The 2.5" drive bay still only has a single SATA connector available on the motherboard, so even though multiple M.2 SSDs would physically fit in the space of a 2.5" drive bay, you wouldn't have a way to connect them all. Even if you only wanted one, I'm not sure there's an M.2 to SATA adapter. I know there are mSATA to SATA, but that's a different physical connector. Also note that even if there WERE an M.2 to SATA adapter, you would need to ensure that your M.2 SSD used the SATA protocol, not NVMe/PCIe, since that would not be able to run over SATA (and even if it could, its performance would suffer significantly by doing so).
Why should I go to the trouble to get the adapters to use an m.2 SSD, when I can get an SSD in the 2.5" SATA-ready form factor ready to drop in?
What am I losing by NOT using an m.2? Is there a way to benefit (gain performance over SATA) from using a NVWe m2. SSD in the 2.5" bay?
There is a separate, M.2 NVMe port on this model - that will take an NVMe drive. The 2.5" drive bays are SATA - they will take only SATA drives (2.5" natively; M.2 with adapter). Unless you already own an M.2 SATA drive, there's no advantage to M.2 over a native 2.5" SATA SSD.