If you need a lot of (not so fast) storage, 3,5" drives are still by far the best TB per €. For a lot of NAS solutions (backups, video/movie/music storage etc.) their performance is completely fine.

Plus, we're most likely talking about Gigabit networking here, so unless your workload consists of very parallel random access, this is going to be the limiting factor anyway.

25 and 100 Gbps is commodity at this point. yes it's a bit of a pain to run fiber in the walls but worth it.