Second of all, it's not just about a decline in speed with him, or it would've really started about 3-4 years ago. A guy at his age is always just one bad stretch and a tight hamstring away from suddenly being out of training for 4-6 weeks, and then keeping fingers crossed that it doesn't reaggravate and turn into 8-12.
I mean, maybe he ends up being Polish Tom Brady and is still banging them at age 35+, but if I were Bayern I wouldn't be betting on it. I think the time for them to start building the next cycle is now. But, again, I'm just a dude who watches on TV.