Other than having to be clever with your roster space, there is no downside to having more dragons. Not one.
You’ll get rewards for new species in your Book of Dragons, you’ll have more places to dump fish and other dragons when you need the resource space, which will lead to more Fishlegs duties completed faster, which means more chests. You’ll have more options for events, which include species, star tier, or color locks, which given more of those much needed resources like fish, eggs and draft tokens.
You’ll get to a point where you are guaranteed to complete a weekend event and get the Premium token at the end, giving you more chances for those dragons. But even then, Legendaries are a <1% chance, so get ready to draft a lot. you want options options, not just mega BP.
Breeding is probably going to give you your best chances at getting the best dragons, since you can pare it down to a single species. Want Murklurker? Best to have two Waveshakers to breed.
What about future unknown hybrids? Sure would be nice to already have the parents of that dragon. Especially two Epics, you could start breeding for the Legendary hybrid right away.
I didn’t start steam rolling until I realized I should be saving and levelling all the unique dragons, not just the Legendaries, not just the higher BP. Getting more experience means higher levels, means more roster space.
Don’t let the 50 level cap fool you it’s not true