It's not completely pointless at all! If sufficiently many users are willing to run a microminer at a small monetary loss, then it becomes practically impossible to run a commercial mining farm. I think we're quite far away from that scenario, but it's a theoretical possibility. I hope that eventually a hundred million people will care enough about decentralisation, and in that case it might happen.
You astutely describe my design for eliminating commercial mining. But I don't rely on them caring about decentralization.
If you can market it as a lottery, it may work. If a billion people spend 10 $/month (on average; richer people spend more, poorer people less) for mining with the tiny chance of 1 in 20000 per year to win a block that pays them a million $, then commercial mining would mine at a loss, but people may still have the incentive to do that. Especially if you tell them that these 10 $ are for a good purpose (to keep their Bitcoins safe). This would also eliminate the need for mining pools.
Not sure if this works out, though. Say half of the cost would be necessary to run a full node (otherwise you don't get decentralization) and the other half would be maintaining costs and electricity to run the hashing asics. Any variation of prices for electricity or on the price of low power asics can mean that commercial miners make some profit (because they would only have the costs for the asics). And 5$/month for a full node may not be enough.