Dragonite is hardly a "perennial" OU. It's historically BL in RBY and even now is still kind of seen as a gimmick, it's BL by name in GSC (and its OU power is very debatable), and it's been overshadowed by Mence in ADV and DPP. As a result, it's decisively BL in ADV and is only considered a top-tier OU threat in recent DPP now that all the better dragons are banned.
As for Egg, there's a far better argument for him. Basically mandatory on an RBY team, and a top-tier offensive threat in GSC. I think the biggest points against it are 1) its extremely sharp drop-off after GSC, which I'd say hurts it enough to be worse than just not existing for two generations; and 2) the variance of its performance in GSC that makes its position as a "top 5" quite debatable.
Say what you will about Blissey being nothing more than setup bait, but it's a known quantity in every match and the best at what it does. Offenses NEED to have a way to deal with it to be successful in ADV and DPP (it's basically an automatic member of most stall teams in these generations), and it was good enough in early GSC to make SkarmBliss a thing (and a moderately successful thing at that), even if the modern bias toward offense in GSC would lead you toward thinking it's total shit (which it isn't, that's hyperbole and you know it).
I'm going to expand on my decision of Raikou at #9; basically, imo it is a shoe-in for that slot. It's got top 3 status in GSC, is one of the most fearsome sweepers of ADV, and is apparently still pretty good in DPP. It doesn't have Eggy's severe drop-off; its peak is way more impressive than Blissey's (Blissey was always just kind of there, never outright defining a metagame but constantly shaping it in some way); and I'm not even going to dignify Dragonite because the fat dragon has no business on this list. Other things it could compare to include Jirachi, which Kou matches or surpasses in peak dominance and which Kou has beat on generation count; Forretress, which has always been a one-trick pony and not even necessarily the best at its job (just the most specialized); and Cloyster, which people seem inclined to dignify like Dragonite even though it faces an Egg-like drop-off in usefulness without ever really being as good as Egg was in RBY/GSC.
I'm more open to #10. However, on the surface, it feels wrong not to include Blissey, but I feel like that's in part because it has the stats and movepool to make it the perennial go-to special wall for generations to come, and not just because of its performance in GSC-DPP. That being said, the only real competitors I see for Blissey's slot are Jirachi and Forretress. Mayyybe Salamence too, although on second thought not really because it's kind of average in RSE whereas its dominance in DPP was very brief and the result of a metagame where tiering decisions were constantly in flux. As for other things people are bringing up, Egg's drop-off is too severe and everything else was never good enough to begin with.