There're still plenty of non-paying players regardless. Or at least, Inno regards the playerbase as healthy enough to continue updating the game.
Like I said, Inno's keeping up with the times. The way the game was monetized in the past is neither attractive nor significant in 2023. All of those old methods are still present, but players today want more to be able to spend on. However, the core concept is the same, and you said it yourself. Spending allows players to do things more easily and/or faster (or as you said, 'in a timely manner'). Costing money now instead of diamonds is, again, keeping up with the times. Something like the Key Master's Workshop is a modern take on something like the Film Studio or any of the other old premium buildings that none of us even know the names of anymore because they haven't been relevant for years. Paying players don't want those old buildings. They want something that actually matters in today's game, and Inno's delivered on that. Still nothing that's 'required' though. Just because it's something you particularly want doesn't mean that it's a bad thing that it requires spending. It means they added something that's actually worth spending on, for the players who choose to do so.
Your mention of diamond use in GE does bring to mind all the complaints about how 'hard' GE4 was when it was first added as well. A lot of the same complaints back then, actually. Diamonds being 'required' for negotiations because of how 'expensive' it was, and the units being 'unbeatable' with their boost levels. Hindsight makes those complaints look pretty silly now, doesn't it?
Also, there's a lot of overlap between GE and GBG participation. The same players making diamonds from GBG are also going to make diamonds in GE, because they most likely aren't using many diamonds in either. The players using diamonds in GE are the ones who're less developed and don't have the resources necessary to avoid spending diamonds, meaning they probably also lack the resources to gain a lot from GBG.
Side note: I think we've done a good job at making this feedback thread absolutely useless with how derailed it's become.