Does it mean I will have access to fewer and fewer M4L devices made by other users if I decide to stay with Live 8?
Inevitably, yes. As more and more users start using Live 9 and M4L (and thus Max 6.1) to make M4L devices the more devices are going to require Live 9 to run. Max 6 has new objects and even new internal coding environments like Gen that aren't available in Max 5. Max 6.1 brings even more features over Max 6.0 including 64-bit support. Many devices will be compatible between Max 5 and Max 6 versions of M4L because it's not imperative for developers to use the new Max 6 features in their devices, but the more people use those new Max 6 features the less this will be the case.
For better or worse it's the price of progress. The advancements in Max 6 make it worth it, IMO.