Sure, but there's a massive difference between bringing in any legacy TNG crew and dedicating an entire season around the whole TNG cast.
If we were just getting a few cameos, or even a Nepenthe-style episode, that would fit with legacy characters being brought in after the show found its feet. Dumping most of the original cast and replacing them with the full TNG cast doesn't need the show to have stood on its own feet - it may as well be a whole new show.
Yes. This. None of what Sir Patrick ever said in interviews pre-PIC ever suggested that they planned to have an entire season dedicated to only the TNG characters. It all sounded more like "we will have some cool guest appearances when we think it's appropriate", like they did with Nepenthe. This whole TNG crew only thing was entirely Terry Matalas' idea (which is why he's gonna be the one who will either get all the hate or all the praise from the fandom, depending on how this turns out). He did say in a recent podcast that he brought the idea up to the ones in charge.
So, no, this wasn't planned from the start. At least not like this. It probably came into play when they hired Matalas as the showrunner. Which is about the same time the narrative in interviews started to shift from "the goal is to eventually find out what the TNG characters are up to and to somehow have them appear at some point but we have to establish the world first" to "yup we will see more TNG characters" to "we will definitely see all of them, an entire season will be dedicated to them and we're gonna dump most of the previous characters for this because a) we can't afford to pay everyone and b) it's also tricky to have ten episodes and a cast that's basically twice as large now as before and make sure everyone gets their scenes".