Since there are so many GPUs currently mining ETH, once the Ethereum transition takes place, there will be a large number of miners looking for the next best option. It seems unlikely that any of these other currencies will grow in value enough to assimilate that amount of hashrate without ruining profitability. If there is no profitable alternative to Ethereum once all Ethereum miners start to change, GPU mining is likely to disappear. In essence, the goal of the difficulty bomb is to force miners and node operators to update their software after a predetermined time has passed, according to Nic Carter, general partner of Castle Island Ventures and co-founder of Coin Metrics.
The idea is that sufficient attention from the mining community could result in an increase in the value of the currency; this is possible, but it may be overestimating the amount of market power miners have. As the crypto craze cooled and desktop miners were hit by ASIC's huge farms, the market was flooded with cheap second-hand GPUs from repentant investors. While these coins are currently profitable to mine, they are not as profitable as Ethereum and have much smaller market capitalizations. Also known as “The Docking”, which plays on the metaphor of the Ethereum spacecraft, forms a segment of the next stage of cryptocurrency evolution towards Eth2. Hypothetically, let's project that ETH prices bounce within a narrow range over the next two years, and that you should be able to mine AFTER achieving ROI in order to create a net profit.
The Ethereum Foundation reminded miners of that inevitability on Monday by unveiling the Kintsugi testnet so that developers can familiarize themselves with Ethereum in a post-merger context. There are other Proof of Work cryptocurrencies that can be mined with consumer hardware to make a profit; however, since many Ethereum miners are looking for new coins to mine, these alternative options may not remain profitable. Of course there are still shitcoins to mine, but once ETH is put to the point of sale, I think many GPU miners will quit. It also helped show how a sudden decline in mining energy won't ruin grid operability; while having more miners increases grid security, it's not critical to keeping the grid running.
The opportunity for miners to maximize their profits before The Merge or identify the cryptocurrency to which they are going to dedicate their computing power afterwards was not said. However, when using consumer graphics cards, there are more opportunities for people to exploit without buying expensive hardware specifically for mining cryptocurrencies. Even if the profits aren't that high, every GPU that works will still generate more money than it costs to run it; since mining is a fairly low effort, I would expect several miners to continue to participate as long as some profits are made. But it seems that the day when enthusiasts no longer have to compete with Ethereum miners for graphics cards is a little closer.