U.2 only makes sense when you 'need' very fast enterprise grade drives (with their associated reliability attributes as well as PLP, but they really are only suited to rack chassis setups, keeping them cool to be rewarded that great reliability that is). For most, M.2 for boot/system and a few sata ssds is perfectly fine, more than enough bandwidth/performance.
In a way, NVME is a bit of a 'look at my amazing low latency and crazy sequential bandwidth' when it comes to most folk. It won't make your computer suddenly run crazy-fast nor will it make your games or web-browsing faster. However for serious creators, ie video editing of 4k/8k footage, NVME makes one hell of a difference once you start working with a fair few layers of raw footage on a timeline, but saying that, even then you can still manage with good old sata if you don't mind the odd lag here and there when timeline scrubbing.
Tech these days especially, is like today's modern cars... most mid to top-end cars are pushing 0-60mph at close to supercar levels, most modern cars are more efficient though some give you crazy performance with the acceptance that they aren't exactly fuel-efficient... but look, you can still jump into your old jelopa and she will still take you wherever you want... not the best of comparisons, but I'm sure there are chaps out there still running scsi320 setups with ide drives.
Latest tech brings bragging rights to most, only the very few are actually in need of the gains from new tech. SAS will be around for years to come, no question about it. In my experience, NVME drives produce a great deal more waste heat, and that is down to how the drives internally do something similar to how raid operates, parallel multi-chip access and all... they're not 'advanced' technology per-se, they just sit on a wider-bus and are using old-hat tech to grab a lot more data to fill that bus up and have a very short path to the CPU with a very simple commandset.
Most consumer motherboards are still SATA orientated with multiple M.2 slots, why? because most consumers spend more on their graphics cards than they do on elaborate storage and network systems... but, by inheriting good old SAS tech from the enterprise world, we get to somewhat affordably roll-out some nice NAS type setups at home, and if keeping that electricity bill down is a priority, then that is where SAS does prevail over NVME, especially when you have 24 of the buggers in a chassis.
I wrote the above after devouring a noodles soaked in chilli... I gotta shoot, my batty is about to do a dragon on me.