I think that companies won't mess with forks if there is not a huge support on the fork, they'll follow the one that is the most supported by the network and the people, e.g. the default Bitcoin.
This is from Blackrock's filling. I assume all other ETFs have a similar take? It is a sort of a disclaimer. "We cannot guarantee that this is the most valuable fork if it splits". Of course you got to realize that all of these are under US jurisdiction, so the government could step in and say, "make this or that fork as the legally abiding one". And there you have it, now all exchanges and miners are forced to mine whatever UScoin fork. This is the problem with the ETFs.
I don't think this will force miners to do anything. Some will mine Bitcoin, some will mine whatever Fork they want.
I think it would have been better if their terms would say the value of any potential Fork is added to the total ETF. I get they don't want to deal with each worthless Fork. I can create Bitcoin LV (Loyce Vision) today and nobody cares. That's okay. But if a Fork has value, they should not just choose the most valuable Fork, they should sell the other Forks and pay dividend to the ETF holders.
Then again, I also get that this could become complicated: what if Bitcoin LV is initially worthless, but after 2 years it's worth $42. That would mean they can still sell the Fork, but the ETF holders at that moment are different than the ETF holders during Fork days. So distributing the divident to the right people is complicated.
When there was a Bitcoin hard fork in 2017 and BCH was created, I remember that many casinos, exchanges and other companies were suggesting people to withdraw bitcoins during the fork because they weren't supporting the fork. Then, after some months or probably after a year, everyone did it differently, they donated people with Bitcoin Cash that was equal to their balance.
I think it isn't and shouldn't be their responsibility to take care of every fork and they won't do that because then they will have to approve the ETF of that fork, just my two cents.