I can also list out multiple common counters to any tier 1 mons, it doesn't make them less powerful.Tapu fini 1->2
Same reason as before but I think I should go into more detail.
Mega gengar, which is extreamly common, can 2HKO it with sludge bomb. Bulu is gaining a lot of popularity and can completely demolish fini, and gain health doing so by using horn leech) Mega metagross can 3HKO it with zen headbutt/iron head, and 2HKO it with thunder punch (if cairied). Zapdos with elec-Z is becoming much more common and it can OHKO fini without a ton of investment. Mega manetric can hit it hard with volt switch, or use snarl in order to lower its already somewhat mediocre special attack. It's 95 base special attack also does give it a hard time taking things out, it can (sometimes) 3HKO AV incineroar with no special def investment with muddy water (If muddy water hits). It will also be 2KOed by KyuBs fusion bolt (although kyub can't switch into fini).
Koko can also OHKO it with tbolt holding orb, and 2HKO it without one.
It also has trouble with a lot of less used mons like kartana, rotom-W, ferro, gene with thunderbolt/energy ball).
Sorry this is a bit of a ramble but hopefully it still gets across my thoughts
Out of all the popular threats you mentioned, only Tapu Bulu can consistently OHKO it with all of its most used sets (and most of Fini's supports OHKO Bulu without much of a problem), despite all of them filling the roll of a Fini check. Another extremely important factor is that not a single one of those can actually switch into Fini without taking too much damage and that's if we don't even take into account the possibility of a Calm Mind or Specs boost.
Regarding some of your points, Koko needs terrain to OHKO fini with LO Tbolt and both Rotom-W and Gene aren't much of a problem at all, since they don't pose enough offense to actually break Fini, especially after a Calm Mind boost.
Also, if anything, Gengar and Bulu's rise only did Fini a favor, but this is a far more in-depth analysis of some of the matches we've seen lately. Do some quick research on how the matchup has been approached by some MetaFini users and you'll see what I'm talking about.
Overall, Tapu Fini is a top tier threat that everyone struggles properly checking and that can easily capitalize on bad teambuilding, while also pressuring the very threats meant to check it that are present on good teams. This is the second time we vote on Fini moving to tier 2 in little over a month and I hope it's the last one in a while.