As we know the more miners the more difficult it gets to mine. So when there are for example 1Million pc's mining it doesn't matter if they are all in one basement or spread each across the world (Technical not fairness or whatsoever related). So the reason why bitcoin mining got so competitive and non usable with a normal desktop pc was because it got harder to mine
Mining got competitive because the reward for mining increased.
If we would switch back to a CPU design algorithm I'm sure as one can possibly be then in less than one year the average joe with its i3 or i5 from the 6th generation won't be able to make a penny a month, look at monero, 10-20 cents per day to keep your CPU at full throttle and this with the coin being up 30% in the last 90 days.
IF we would be mining only via CPU farms would still exist, you wouldn't be able to compete with huge datacenters that could at any time program their servers to mine when they are not used by other applications not even going to mention botnets.
That being said I don't see mining dying down completely for small miners, they have one huge advantage over farms if they manage to get cheap electricity, you don't pay maintenance, you don't pay rent or employees and in most cases, you don't pay tax. If you live in an area with cheap power or you have excess power from some already built generator then you can compete with any large farm. Besides, a small miner could run its gear at a loss just to secure the network or simply out of spite, paying a few tens of dollars a month, a business that owns a farm won't be able to run like this long before going bankrupt.