I'm guessing the biggest reason is that Ethan wants to shut down is that he doesn't want to deal with the stress of making sure the balances of all investors (BTC bankroll alone was > 100 BTC, or roughly 5 million USD at current rates) are safe. Continuing with altcoins likely wouldn't solve that issue, especially since ETH & LTC are fairly major coins too.
I agree with you on that, as i said, or tried to, the same reason just-dice switched from btc to clam, as i was told dooglus didnt wanted to take the responsiblity anymore so he switched to a coin with "much" less value, cause people did not want to see the site dying.
If not with ltc or eth, i think there are still altcoins (doge, polkadot, etc) which could be used instead, from what i understood most people love the community of the site and the gui and all other things around that, to abandon it completely would be disappointing, even it is all ethans decision.
Back in those days just-dice changed because Canadian rules changed, doog and his wife was managing basically a small time place, it was so small that I remember clearly that we were chatting with doog and his wife in the chat, there were few more people as well, bayareacoin was there too if I am not wrong, so basically like a 10 people situation where we were always there and some people came in and go out but that 10 or so people were always the same.
So, when the change happened we talked about it and he specifically said it was legal reasons, because it was "bitcoin" that had a new rule but crypto didn't, so he had to pick another coin and clam was nowhere in any law so he switched to clam knowing that casino wouldn't be as big as it could be with bitcoin, but at least he would not have any legal problems thanks to the switch, which is why I think his was a bit different than just stress.