Manannan, it's not a basic game mechanic that has been broken. Buildings are either buyable, or they're not. These are both valid states for a building in the game.
It absolutely is a basic mechanic. Buildings can be purchased with energy/gold has been a basic mechanic for some time in the Civ series as a matter of fact. Deciding "we're making this very specific building unbuyable because we fudged the balance really bad" is taking out a mechanic because you're unsure of how to balance something else, plain and simple.
You have to look at the why. Why are Wonders limited? Because they're powerful. Because in-lore they're unique. Two reasons, one rooted in design. But that doesn't explain why you can't buy them. You can't buy them purely for balance reasons. There are absolutely no lore reasons why this is the case.
No, you can't buy wonders, but we're not talking about wonders, we are talking about run of the mill every town is gonna (or at least can) build one of these things buildings, that can be bought. If this was CiV and they said "this one particular wonder is too strong, from now on you cannot bulb it with a great engineer while this one over here is kinda weak so that one can be bought" what you're saying might kinda be related to what we're talking about, but we aren't talking about that hypothetical situation, we're talking about something else entirely.
No, not ergo. The sky is blue, ergo so are some flowers. You're talking about two entirely unrelated things.
we've established a precedent for limiting buildings based on power. The Trade Depot is powerful. Thus, balancing it via this method is viable and makes sense within the game.
No. We have established a precedent that wonders cannot be bought with money. The precedent for buildings is that you can buy them with energy. Further, even if "they're too powerful, you cannot buy them anymore" was an actual precedent in the series, trade depots are not powerful, they are a building that allow you to build a unit that is powerful. The logical thing to do would be to actually try to balance trade routes instead of ham-handedly changing a basic mechanic and hoping it results in balance. I mean honestly, what's the logical conclusion to that line of thinking? "The Augmentary is really powerful with it's one quest option activated, so you can't buy it anymore you have to hard build it" instead of just changing the quest reward?
Whether or not you agree the change is balanced is another debate entirely.
Sure (for the record I view the game as still being horribly unbalanced), but you haven't asked if it actually balances the game yet. My point is still that it's inelegant (ie: bad) game design to ignore the mechanic that is actually broken, and instead break another mechanic to try to fix the problem. Again, I'm pretty sure that's the definition of inelegant game design.
On the subject of the Mind Stem, this further exposes your motivations. You care far less about Wonders bring unique than you do buildings being buyable, which showcases that this is a personally motivated decision (i.e. by how much you care about the mechanic as supposed to actual arguments about the design and purpose of the constraint).
No. I care less about mind stems primarily because they were a less broken feature in the game than trade routes were/are, but also because I can see an argument being made for them being classified as a "wonder", or more accurately to classify them as whatever spaceship parts were in CiV if I need to make this a bit more clear for you. There's an actual precedent there since you're so eager to show one. The better solution is still to rework the mechanic that is actually broken (the victory condition itself, and how spammed cities make it too easy to complete rapidly by spamming mind stems/xeno sanctuaries) but I'm more willing to allow them this non-sense. That is not confirmation bias (which I think is the term you were looking for), try again.
What if someone else placed as much importance on Wonders being unique, with your Mind Stem suggestion? Who would be right?
The answer is nobody because you're debating based on a personal opinion.
No, this is not an entirely subjective debate, we are not debating whether or not we prefer Justin Bieber or Taylor Swift (the answer clearly is <3 both equally before anyone asks). We are not making up definitions as we go along, we are talking about things that are tangible or at least definable and exist in a reality we co-habit. Sorry, but you don't get to throw your arms up in the air and declare "well that's just like, your opinion man" on this one. If we were asking "does this change make the game more fun" then by all means, that is personal opinion that I can't argue with you over but we aren't; we're asking if the game designers have done a good job in trying to balance their game.
