Yeah. The GOAT is getting replaced. And there is no replacement.
But you know where the GOAT is also not coaching? Iowa. Or Auburn. Or A&M. Or Texas.
It's not like Saban left for Colorado and all his players are transferring to follow him. He's retiring. And they're leaving, but not going to schools with the GOAT coach because, as you say, there is no replacement.
I don't think it's unreasonable for a player to be concerned about the uncertainty with a brand new coaching staff coming in, even if those coaches were successful elsewhere. The coaching landscape is littered with coaches that were good at one place, and then failed when they went somewhere "bigger." In fact, just looking at the blue bloods over the past 20 years or so, I'd say the outcome of failure is far more likely than a new coach coming in from a "lesser" job, and being particularly successful.
After losing their last "good one" for whatever reasons, Texas, Michigan, USC, Notre Dame, Nebraska, Tennessee, all turned over coach after coach after coach, trying to get back on the right side of success. And before Saban, Alabama turned over a few as well. Really only OU and Ohio State have had much success replacing a good coach with another good coach during that span.
So knowing that the odds are the replacement for Saban is, at best, not going to be wildly successful and, quite likely, is going to fail-- I can see why kids would want to leave for a more stable situation at Texas or wherever. But it definitely doesn't explain why you'd transfer to a school that is also in the midst of a complete coaching changeover like Texas A&M. NIL could be influencing that.