Looks like you are confused, the price of the coin doesn't affect the chance of a 51% attack, it would affect if the coin were Pos like ETH where some one who holds 51% of the coins can do de attack with staking, but in bitcoin and Pos coins the attack comes from the miners.
Honestly, you got me confused now. Either I do not get it just like OP did not or we confused each other.
Is OP not partially correct? It was much less costly to provoke a 51 percent attack on Bitcoin back when Bitcoin was so much cheaper. Though the Price does not have a direct relationship to how successful or possible an attack is, I think it is much less likely to provoke one as long as the Price of Bitcoin is up and bound to increase. Because implicitly, it means a lot more interest is gained and I do not think we have ever had a Bull Run with out having a significant increase of the Hash Rate at the same time.
It is curious even. The higher the demand, the less possible it is to attack Bitcoin. And similarly, the higher the demand, the more likely it is for bad actors to want and try to find ways to attack it. So as it is always said, Bitcoin truly is becoming more and more Secure the more Full Nodes, Miners and Users we have. I always found this fascinating to say the least.
You are correct Sir that the higher the demand,the less possible it will be to attack Bitcoin despite the bad actors too that will arise, I think from 20 years and above that may be after more than 4 BTC halving, we could dig in deep and predict with a little more precision in Bitcoin and it's unfolding potentials and benefits, but for now let's keep enjoying it and keep hope alive.