There are ~10 million GPUs mining cryptocurrencies in the world today. Because the majority of them mine Ethereum (5-10 million are needed to generate 250 TH/s), it means the other coins can easily be overpowered and attacked if a small fraction of GPU miners decided to do so. For example look at Monero: 400 MH/s of CryptoNightV7 hashrate means there are 400k-800k GPUs behind it, therefore only 4-8% of the pre-existing worldwide GPU mining capacity is needed to attack it.
In a way we could say Monero is vulnerable to a "4-8% attack!"
The best way to defend a coin from this scenario is for it to implement a unique PoW algorithm that is very ASIC-friendly so that GPU miners couldn't overpower it. Of course the same attack scenario would exist if there is a large pre-existing installed based of this ASIC, so the PoW algorithm must be unique. For example Bitcoin Cash could currently be attacked by 10% of Bitcoin miners as they are both mined by the same SHA-256 ASICs.
The irony is that the misguided trend to try to be "ASIC-resistant" is actually worsening the value proposition of all these GPU-mined coins as it makes them more vulnerable to the very real possibility of 51% attacks...
Whichever fab produced it would have a huge hardware advantage in mining. They would mine themselves and just use 51% of their physical units (versus renting on a time-slot market) to do the attack.
The price-tank disincentive is multiplied by the block reward size -- the lower the reward, the the less the miner would care. The price-tank disincentive also is attenuated by future equilibrium mining. If you make the first ASIC and don't attack, future manufacturers will pop up as mining becomes more profitable. If you tank the price, mining profits may become low, but you thwart competition.
Attack gains are not connected strongly to the above disincentives. Attack gains are higher the more often you can cycle the coin -- you can actually steal way more than the market cap of the coin theoretically by cycling exchanges / other mechanisms of payment. Then you start running your 51% attack and reverse hours, days, weeks of transactions.
I see your point that there is some disincentive for a 51% attack, but I'm not sure that's enough.
This is exactly what they are doing.
Lots of people are buying GPUs, but any SINGLE gpu coin has a very small percentage of total GPU hashpower.
For the extreme case, image that I have GPU coin, but I am only supporting it with MY GPU.
That means that any other person in the world, can now attack my coin, if they have more GPUs than me (1).
The fact that lots of people own GPUs doesn't protect me, because those GPUs aren't contributing to the network!