This is the question:
How long is the game?
And this is the answer:
The cost is subsidised by the block reward.
So, the game will be over when the cost is more than block reward.
Do not count the "cost" in dollars. Count it in joules or kwh.
After it we will see decreasing hashrate and ongoing decreasing the security of confirmed transactions.
All the factors which allowed bitcoin system to grow in past will push the system down.
Today you can easy reorganize the blockchain in some dead altcoin with an obsolete asic.
Tomorrow you will be able to reorganize bitcoin blockchain with your currently running asic.
Somebody definitely will "test" is because "he will be able to do it with small efforts"
I repeat: I think not about today. Like a chess-master I think several turns in future.