Mount isn't going to be forced out. That would be ridiculous. He's only going to go if he wants to. The whole reason this show exists in the first place is because of the obvious once-in-a-generation chemistry between him, Romijn, and Peck. However, shit happens. Bab 5 is the perfect example. The only reason there was a change in leads was because O'Hare had a personal tragedy. Had that never been an issue, Tron with a box and lighter probably would have stayed home. But tragedy or unexpected happenstance isn't the only reason for someone to change their mind about a project or want a premature exit.
But that doesn't exclude the introduction of Jimmy K. Those two things are not mutually exclusive.
A lot has happened since the end of Disco season 2 -- not the least of which being that global existential crisis thingy. Besides having a permeant effect on the general approach to production, it also greatly accelerated the growth of the streaming market, meaning CBS is facing much stiffer competition for the market share pie. Also, the last clinging hopes of there ever maybe, possibly, perhaps being more Kelvin films has all but completely fizzled out.
Also in the timeframe, Yeoh has forged a second wind to her career, which is outstanding but leaves little time for Star Trek in her schedule. Despite the pretense it's still in development, it's impossible to say whether her show happens or not. But even if it does, it most likely won't be in whatever form was initially planned. (Probably a limited run miniseries.)
Not only does this leave a gap in the production schedule, but it also leaves a gap in market draw and whatever pop culture significance there was to be had.
Enter the other half of the franchise's most famous -- and most zeitgeist-relevant -- duo.