People used to search for extraterrestrials' signals just for the fun of it using SETI project on their computers 15 years ago and weren't paid a cent for that (maybe they still do, I haven't followed it since). Imagine that there is at least a small reward that you can get from running a mining app on a cheapest laptop, in short, there will never be a lack of miners, even if the reward is miniscule in the following years, the transaction network will always be there and it will be be extremely distributed, which is the crucial factor to a crypto survival. My view of it, feel free to disagree.
It will always cost electricity to mine and create heat and noise. So if the security of the network depends on a few people who let their computer running for the fun of it ... then it's time to get the popcorn.
You know those botnets that turned out to be useless for bitcoin because cpu mining was not efficient?