If your conditions are set wide enough you can prove just about anything.But can't you easily argue that something that is damaging to balance is damaging to diversity? Let's look at the lists that had 6 wins or more at worlds. 10 rebel lists, 12 Imperial, and 19 Scum.To conclude, things get nerfed because they distort the metagame: not because they're damaging to balance but because they're damaging to diversity. Things get errata nerfs if they're killing off a large number of strategies and builds that would otherwise be viable. Powerful generalist builds that don't hard counter things out of the meta tend to be dealt with by new releases rather than rule changes.
Rebel lists without regen or TLT- 40%
Scum lists without Zuckuss or Manaroo- 37%
(side note: 2 of these lists were quad TLT, and another had 2 TLT Ys. If TLT is included in the restriction, the % plummets to 21% )
Imperial list without Palpatine or x7- 0%.
Yes, perhaps balance wise, regen, Manaroo, TLT and Zuckuss are a little overpowered, but they largely have not eliminated faction diversity to the extent FFG needs to nerf them... Yet. If Dengaroo overwhelms this regional season though, I think Manaroo may get a look.
Unfortunately, Imperials cannot say the same as its two rival factions. They are being warped by both Palpatine and x7 in a very real way. Even GOOD lists, such as Crack Swarms, are being lost because they can't compete with the efficiency of lists including these upgrades.
If any cards need to be looked at next, it's Palpatine and x7. Not because they destroy diversity of other factions (as Uboats did to Rebels), but because they destroy diversity of their own.
For Rebels the "TLT or regen" condition would include lists as varied as Corran+Miranda, Kanan+Ezra+Biggs, Dash+Poe and say, Triple ARCs. What do these list have in common? They play differently, they fly differently, they have massively different preference for certain match ups. I'm honestly surprised 40% of rebel lists do NOT fall in this extremely broad category.
Same goes for other factions:
For Scum "Zuckuss or Manaroo" would include Dengaroo, Manny + double fangs and Bossk+Dengar.
For Imperials "x7 or Palp" might include DeciWhisper, Crackglaives, but also Soontir+Inquisitor+Palp Shuttle.
So yeah, some upgrade cards are effective and popular. A large percentage of successful lists do use at least one of them. That does not mean the game lacks diversity. In fact, the only list in the Worlds that did repeat itself several times would be Commonwealth Defenders. And yeah, they're popular, they won Canadian Nationals, they're straightforward, effective and fairly easy to fly. But somehow they didn't dominate the top, did they? And apart from that popular netlist everything else was just one huge surprise for me. This year's Worlds were probably more diverse than ever. We even had a Han+Jake in top 4 for christ's sake! I wouldn't believe it if someone told me that list could make it to top 4 at regionals in this day and age.
So you did not see Dengaroo and Manaroo Mindlink lists coming? Or the general comeback of TLT and rebel regen? I mean the runner up list was exactly this, even if it was in a surprising fashion, i'll give you that!
Nothing bad in this, but i could see these lists come from miles away on worlds and guessed Dengaroo world champion right before the first list was even posted.