Hey there, welcome back no apologizes necessary as those are some legitimate concerns. So I've been boxin for a few months now and here are some of my thoughts on it.

1. 6 years ago so that's about beginning of wotlk through ulduar, and I agree those dungeons required far less movement and coordination. In WoD, for the most part, I'd have to say dungeons are not as friendly to boxin as they once were, however it doesnt mean they are not doable. I never box'd in wotlk, I had a friend that ran a prot pal and 4x ele shams and he facerolled everything then. Today that team is pretty much garbage as I have learned the hard way lol. There is enough movement in dungeons to render ele sham dps weak compared to say hunters. Some fights in WoD dungeons require constant moving and coordination, and they can be quite annoying, especially with an all melee team. The good news is, this late into the expansion you can easily gear out your fresh 100s and steamroll through basically every WoD dungeon through heroic.

Mythic dungeons are another animal. At this moment I haven't been able to box and clear an entire mythic dungeon even with a ilvl 730+ tank, but I have been successful against ~8 mythic dungeon bosses scattered across the 8 or so dungeons. The best 5man team to box and be able to clear almost all 5 man content, in my opinion, is blood dk (or prot pal, I prefer prot pal), disc priest, and 3x bm hunters. There are some decent lazy macros out there for bm hunter dps rotation so its pretty much plug and play. Another thing is Timewalking dungeons and they are really fun, basically reliving old dungeons from past expansions, not entirely how they once were but still good enough to bring back those memories. Some of them can be challenging at first since your ilvl is scaled down to a flat number, however you can easily obtain a bunch of socketed gear from old raids, perhaps even dust off that old legendary sitting in the bank (or even complete some) you can certainly have a good time doing these. What's even more fun is if you can get your hands on some old tier bonuses like hunter/dk tier15 from throne of thunder and just melt face lol. I made a 4xdk,1x disc team specifically for timewalking, the dks are in t15 and just a few weeks away from 4xshadowmourns, and it is definitely fun (although I am a bit bummed the mount dropped before I could complete the legs ><).

2. I agree with JohnGabriel, I haven't had any problems with boxin so far (non pvp) as a result of blizzard's doing. I'd say more movement and mechanics in dungeons is inhibiting enough.

3. I can't account on how fun boxing was before WoD but I am having a blast. A few months ago I was just about ready to take a break and wait for legion to drop but decided to give boxing a try as it peaked my interest and I don't regret it one bit. Sure there is a bit of a learning curve at first but no problem that can't be remedied here or via google search.

4. Yes, a max level 5 man team can quite easily clear old raids (excluding maybe 1 or 2 simply because of mechanics) and some old raids are a decent way to make some fast coin.

Hope this helped =)