Seriously;using latios is immensely stressful, because you constantly have to watch out for being pursuited, and often you will be randomly killed for no good reason. Hydreigon lacks this problem, and has fire blast to remove other special walls. All latios would need to be a massive threat is those two things-oh yeah, and STAB dark pulse for jellicent/reuiniclus.
Using latios is indeed not as easy as most of us thought back during round 2 (spam specs draco meteor) and hydriegon doesn't have the problem of running into random pursuit users.
I think I've made my piece but I'd just like to mention that we'd probably still have Blaziken, Garchomp, and Manaphy in OU if we went through the trouble of countering them as much as we have Excadrill.
Aside from garchomp I agree. Garchomp wasn't banned thanks to counters; he was banned thanks to sand veil hax.
Besides, blaziken's counters are MUCH sparser, especially in its beneficial weather.
True as this may be, we don't know what the metagame would have turned out like with blaziken still around. If you told me that 252/252 SpD tyranitar would be one of his most popular sets back during gen 4, I would have laughed at you. And yet, here we are. What's to say players wouldn't end up using 252/252 bold starmie as a means to counter blaziken? (isn't ohko'd by anything save for +2 adamant shadow claw and has trouble with mixken). Dealing with blaze mainly entailed figuring out what set he was running. Or packing heavy priority.