I have been a long-time member of NikonCafe and for something like 30 years always had been a loyal Nikon user, so of course when they announced that they were (finally) getting into the mirrorless arena, I, like many of my Nikon friends, was excited and interested. Unfortunately Nikon (a) was resting on its perceived laurels for much too long and (b) was way, way too late to the parade when mirrorless was becoming a really significant entity in the photography industry. Nikon apparently thought that, OK, they'd jump in with mirrorless and offer their own adapter and make promises which weren't realistic and that would be that. Not so. When reality bit at least for me, it really crunched down hard and I was actually stunned but not surprised at how disappointed I was and how, even as I knew that Nikon wasn't going to be meeting my needs in the mirrorless realm, there was a lot of sadness that I felt. Actually, also, too, right along with the disappointment and the sadness there was and remains some anger as well, and that has continued to linger, even two years on..... And, yes, time to let go of that, right...... I know I'm not alone, too, as many other former Nikon users are now using Sony or Canon instead. Nikon missed the boat big-time on this whole mirrorless thing and even though they're now scrambling to catch up, IMHO it's really too late. They've lost me and lost a bunch of other people who once considered themselves Nikon users for life.....
My Sony gear has actually far surpassed my expectations and goes beyond the results that I used to get with my Nikon gear and that in itself says a lot.
I know that I am far from alone in this, that there have been many former Nikon users who have made the switch to either Sony or Canon for their movement forward with mirrorless camera bodies and lenses. Sony has been in the lead in the mirrorless game for quite some time, and that's not surprising, especially as they do actually have good gear to offer right now, plus continuing to come out with outstanding new lenses and bodies. People interested in Sony do not need to just look at something listed in another manufacturer's "road map" for the future and wonder when they'd be able to buy and use that body, that lens.....
And, yes, in the beginning Sony, even as they were working on developing their own lenses also had arrangements with Zeiss and others to get some additional lenses up and running and into the hands of customers so that there WOULD be something in the way of lenses for people to use with their new FF bodies right from the get-go, and also had no problem with third-party manufacturers also using the Sony E-mount and reverse-engineering as needed in order to also provide more lenses as well.
It really wasn't until the Sony A7 III that things really took off for them, though, and that camera body has been a huge seller for Sony. In the meantime, Canon and Nikon were still piddling around with DSLRs and not seeing that they needed to be moving into the mirrorless world sooner rather than later. Canon has fared better than Nikon, though, as at least they actually did get themselves in gear (play on words intended here!) once they saw the handwriting on the wall......and with Canon's huge customer base, that was a very wise move. The flexibility with ease in using adapters also has made a difference, too, for Canon users.