It's all one big connected market with hashpower. No (established) coin will be significantly more profitable than others. If it is, miners will switch to that coin, difficulty increases and profit goes down until it reaches equilibrium with other coins. Much of this happens automatically today due to profit switching software and large services like Nicehash.
ASICS however are not equal in this connected market because they can only do one thing. So when Monero forked, they took all that hashpower off the market. (There are still some coins for them but soon there will not be.) This resulted in increased profit for GPU miners on all currencies. First Monero but then it spread to the entire market as a result. There will be a similar effect when ETN, DERO etc. forks away but not as large.
Then, there can be the opposite effect. For example: If/when Etherium goes POS and takes away mining, most of the GPU hashrate on the planet will try to switch to something else.
Guess what they will switch to? First other ethhash algos but then Cryptonight first and then Equihash as a distant third option. This will totally kill profit for the entire mining market and most miners will have to quit.