BuildCraft's objective is to place the player in the shoes of a factory designer with limited pieces and to make the best of these to create the best they can. It's as sandbox as sandbox gets, even more so by the lack of progression in it, you're playing an open sandbox, not a linear(-branching) game. A thing that seems shocking to many, for some reason, is that mods need not cater to every user, and catering to a niche userbase is perfectly fine. It doesn't make the mod worse, it just takes it in a different direction.
Quite a lot of the so called "buidcraft replacement" mods don't replace buildcraft, they actually do something different. Mods like EnderIO with its plug-and-play low effort transport systems, MineFactoryReloaded with its "magic block" farms or even Thermal Expansion with its 100% safe, instant transport energy system are doing something different from what BC is. They're not trying to put the player in a position to design complex machinery, they actively simplify the act of making these machines. Some more or less, of course. The common method of "balancing" for simplifying the construction process is to complexify the crafting process. So to make a block that works for you, you end up having to do 10+ chained crafting recipes, not for too long at least, because Applied Energistics takes care of making crafting simpler, so it eventually becomes a game of resource management... which, with the ability to generate insane amounts of RF with systems such as Big Reactors, stops being management and ends up being hoarding. And sheer resource mass is utterly useless if you have nothing to use it on, and eventually, after you build (in a crafting menu, possibly even an ME interface, with little world interaction) all of your farms and systems that have been designed and function out of the box thanks to the mod developers' algorithms, you got nothing to use it for. And you're not proud of that. You're not proud of having figured out a great system to do <thing>. You're not proud of all the time you spent finding the chink in the limitations that'd let you do what you want. You just went through the content, climbed the tech trees, amassed the resources and are ready to call it a day for that world and start a new one.
And that's not inherently a bad thing. There's, as evidenced by the popularity of these mods, a very large amount of people who enjoy this playstyle. But that's the thing, BuildCraft does not cater to this playstyle. And making it lean towards this playstyle won't make the mod better, it'll turn it into a frankenstein of its former self who was built with the intent to go in the opposite direction as its primary objective and goal. That objective is what it should strive towards. The better it can make the gameplay for the people who adhere to the playstyle it caters to, the better it is. A mod that is excellent for one playstyle is better than a mod that is ok for all playstyles, in my opinion, and these niches are good for those who do not play in the conventional "tech tree craft stuff see cool items amass resources get bored close world" playstyle.
So no, these so called "buildcraft replacements" are actually going in the very opposite direction, and if you find that they replace buildcraft for you then you're not the target demographic for it. Is buildcraft antiquated? Sure, it's old, it uses some techniques that existed back in the day. Is it underpowered? Yeah, considering how powerful the average mod is now. Does that make the mod bad? No, it makes it different.
Quite a lot of the so called "buidcraft replacement" mods don't replace buildcraft, they actually do something different. Mods like EnderIO with its plug-and-play low effort transport systems, MineFactoryReloaded with its "magic block" farms or even Thermal Expansion with its 100% safe, instant transport energy system are doing something different from what BC is. They're not trying to put the player in a position to design complex machinery, they actively simplify the act of making these machines. Some more or less, of course. The common method of "balancing" for simplifying the construction process is to complexify the crafting process. So to make a block that works for you, you end up having to do 10+ chained crafting recipes, not for too long at least, because Applied Energistics takes care of making crafting simpler, so it eventually becomes a game of resource management... which, with the ability to generate insane amounts of RF with systems such as Big Reactors, stops being management and ends up being hoarding. And sheer resource mass is utterly useless if you have nothing to use it on, and eventually, after you build (in a crafting menu, possibly even an ME interface, with little world interaction) all of your farms and systems that have been designed and function out of the box thanks to the mod developers' algorithms, you got nothing to use it for. And you're not proud of that. You're not proud of having figured out a great system to do <thing>. You're not proud of all the time you spent finding the chink in the limitations that'd let you do what you want. You just went through the content, climbed the tech trees, amassed the resources and are ready to call it a day for that world and start a new one.
And that's not inherently a bad thing. There's, as evidenced by the popularity of these mods, a very large amount of people who enjoy this playstyle. But that's the thing, BuildCraft does not cater to this playstyle. And making it lean towards this playstyle won't make the mod better, it'll turn it into a frankenstein of its former self who was built with the intent to go in the opposite direction as its primary objective and goal. That objective is what it should strive towards. The better it can make the gameplay for the people who adhere to the playstyle it caters to, the better it is. A mod that is excellent for one playstyle is better than a mod that is ok for all playstyles, in my opinion, and these niches are good for those who do not play in the conventional "tech tree craft stuff see cool items amass resources get bored close world" playstyle.
So no, these so called "buildcraft replacements" are actually going in the very opposite direction, and if you find that they replace buildcraft for you then you're not the target demographic for it. Is buildcraft antiquated? Sure, it's old, it uses some techniques that existed back in the day. Is it underpowered? Yeah, considering how powerful the average mod is now. Does that make the mod bad? No, it makes it different.