Mate, if they're a hybrid then they're hardly either of the species anymore (which is why hybrids aren't considered true species).
Though I dont get you. Are you saying the quality of the cross decreases when crossed back to a pure? Nevermind that it's not always true (many breeders cross a hybrid back to a pure specifically to obtain 'better' traits), but well what does that have to do with the quality of the pure? If you say they're crossed back and the purebreed has a decreased quality, then again, they're hardly pure anymore. They're hybrids. And even if they are, it doesn't stop other people (or even the same people) to coninue breeding purebreds.
If we have 10 quality Firemouth and use 1 to crossbreed, you still have 9 to linebreed... those 9 don't suddenly become worst in quality. And if I breed that one Firemouth with whatever other species, whilst you have 2 pairs (4 pairs possibly, but let's go with a more conservative number) then you're still producing on average twice the number of quality Firemouth to my hybrids... and then if I decide to take one of my hybrids to breed with one of your offsprings, then again, we still have heaps of quality Firemouth around. Those Firemouth don't suddenly turn drab or become super deformed when one of their sibling is crossbreed somewhere else...
If one keep and breed pure fish, they will always be pure fish. They wont suddenly turn un-pure if their siblings elsewhere are crossed.
Lack of quality comes from lack of selective breeding, not crossbreeding.
The original example that sparked this is in fact an example of this. Texans/Green Texans may be comminly crossed, so are Red Devils/Midas or other similar looking species, but there are few hybrids of Salvini around in Australia. I know because I love my hybrids just as much as purebreds and looked all over for Salvini hybrids and unlike other interesting species that could be hybridized, it's hard to find mention of Salvini hybrids in Australia. Salvinis are an example of a fish that DOESN'T seem to attribute most of its lost of quality to crossbreeding.
Yes, using a fish for crossbreeding purposes removes it 'out of play' and there could be one less quality fish to linebred. However, there are so many more reasons. People keeping quality fish is a large factor, as that essentially prohibits the fish from contibuting its genes in any way. When it comes to trying to breed quality fish, worse are the one-sex display tanks. These prevents a large number of fish from being able to breed. Then there's the spread of low quality fish. Sometimes I am at fault here, as I don't like to kill my fish. Au contraire, I don't breed my 'low quality' fish either. They're for my personal entertainment. High quality are for other purposes (distribution).
So back to the 10 Firemouths example. I may take one to crossbreed. Bob may take 3 for his display tank. Sam may take one to keep with a low quality fish coz he don't care. In fact, given the number of casual fish keepers out there, it's more like Maria take another to keep with a low quality fish as well. So there's four left for you to breed high quality Firemouth from, and if you're lucky, you get a pair. You probably instead will only get one, or if you're unlucky, none.
And so suddenly starting with 10 high quality fish, we only have a much smaller number producing more high quality fish. Or in the worse case scenario, none, and we have to linebreed from a lower/mid quality stock.
So dont tell me crossbreeding is the source of lower quality fish.
P.S. If you disclude the hybrids from the species, then they will indeed have a smaller gene pool, but are no less pure. If you include hybrids with the species, then the species will be less pure but the gene pool will be larger.