Bitcoin or ETH is ultimately similar value just in different numbers, same as any currency, euro or Yen doesnt matter if the number is high. Its also how easily I can get hold of that currency, more miners are in ETH so really it should be more available and people could just gamble what they mine.
You analogy is on the place, but what stops someone to adjust bets accordingly with his bankroll. You know how much you have, and with odds you know how much you expect in return. If betting with
bitcoins is more expensive, then profit is higher in the end.
It is not matter with which currency you are gambling, in gambling odds are more important and with that odds you calculate your possible profit when you multiply it with your bet. Expensive gambling
for me is everything over 100 euros, that is 0.2 BTC, tomorrow that can be 0.05 BTC, this is limit that everyone of us have here, for some people 100 euros is nothing, just a first bet.