I think the Pany/Olympus experience of adapting their earlier 4/3 SLR lenses to m4/3 is a more valuable history lesson. They did an excellent job of engineering their new mirrorless m4/3 mount to offer the best mirrorless usage while maintaining "full" compatibility with adapted 4/3 dSLR lenses. I put "full" in quotes since the AF performance of the adapted lenses often left much to be desired. The AF systems are of necessity quite different and the dSLR lenses were not optimized for working with the m4/3's focusing logic.
When µ4/3 first came out there was no way to get phase detect autofocusing (PDAF) to work. Contrast-detect autofocusing (CDAF) was born out of necessity, but as you say, the lenses needed to be reworked a bit to support it and make it work well. But eventually Olympus introduced their flagship line (the E-M1 series) that included PDAF on the imaging sensor, itself. Sony also uses this method. So as far as adapting older lenses, it's less of an issue now than it was when 4/3 began to transition into µ4/3.
3. Mirrorless cameras suck batteries. I'd have to go nuts to run down a battery in my D800 in a day, while an A7 or A9 user needs to plan on carrying a few batteries with them.
It's true that a mirrorless camera will consume more power than a standard DSLR, but I think the concern about batteries may be a bit overstated, depending on the camera model. I use an E-M1 Mk2, and recently did an informal event shoot. I shot over about 6-7 hours, hitting somewhere between 400-500 photos as well as a few minutes of video. My lens of choice was one of the few with an in-lens image stabilizer, which
theoretically has an increased power draw, as well. Yet by the end of it, the first battery was drained and the camera had
just switched over to the second battery. I was pretty surprised that I made it all on a single battery.
I've heard that some of the Sony models fare far worse, and I can't really comment on that...
When I first transitioned to mirrorless I admit that I had battery anxiety, but after a bit over a year of shooting with it, those fears have subsided. Put another way, I had four batteries for my DSLR (an Olympus E-3, which took two batteries into the add-on grip at a time), but feel fine with only two for my E-M1 Mk2 (which is more of a pain to load two batteries into). I admit that I've become more conscious about turning the camera off, which is something I don't recall worrying about with my DSLRs, but it's so quick to start up that flicking it back on is rarely an issue. (The camera intelligently turns the EVF on and off based on its IR sensor, but as I often have the camera slung over one shoulder, sometimes my torso can trigger the EVF on, which needlessly burns battery.)