New alt coins have different structure and mobile mining is easily possible.
New phones like Xperia ZS are way more powerful than lots of PC's 4-5 years old.
Plus there is always the android emulator for pc if needed more power
Like others and you have said, there's always PC emulation for more power. Since money is money, there will be people who will try to maximize their gains by probably using multicore server processors like the Xeon series to completely out compete the performance of mobile phone. Plus if there's just a massive flood of mobile users in the millions, what kind of profit will they earn? Won't this cause inflation?
You've also said that it's incorrect to compare bitcoin/etherum mining to alt coin mining due to difficulty. Sure it may be easier but wouldn't it just cause a massive flood of tokens which ultimately drives to price to nothing (again inflation)?
I think tackling the microtransaction industry of mobile gaming with cryptocurrency is interesting though as lots of money is already being pumped by mobile gamers.