Well, first off, I disagree with you that a blue deck should be able to do all of those things, or at least all of those things well. In this Utopia (secular) where colors are balanced, there's trade off, so blue would be able to do control and combo with help, but not aggro (4 referenced offenders named, old fish is fine). Just like currently, green does aggro, lil combo (elves), but not much control (
city of solitude,
elephant grass, and?). This excludes obvious printed hate like
choke,
combust, red/blue blasts,
karma,
gloom,
lifeforce,
deathgrip.
Saying conditional cards are unplayable is a fact of life. I'd love it if some cards in other cards were playable, but for circumstances they aren't. There's no panacea like b'storm in other colors to just make these fringe cards playable. The gripe lots of people have with bstorm is the opportunity costs it eliminates for these conditional cards, such as Stifle.
Tangent on example of the given conditional card Stifle:
Stifle is used as a 1-mana
Stone Rain more often than not. If it were used to nullify a Griz trigger, or Top Spin, or storm trigger, it'd be fine and used as the corner case sb card that makes sense. But because it's blue, pitches to FoW, and there's no penalty for running it, spam everywhere.
Bind isn't as versatile, costs 1 more, but replaces itself and gets no play. If that were a blue card, I'm sure it would see play in Legacy, just like
Squelch sees (some) play in Modern. Stone Rain is not a playable card (unless you wanna get cute and break a
Energy Field). Why does the LD color have an unplayable hallmark card, but the library manipulation color get a fetch-wasteland killer for U? Oh, because new players don't like LD, LD isn't fun, blah blah blah. But blue is now the best LD color.
A lot of the argument is based on blue has
almost all of the toys, and blue mages want to keep them. Since WotC hasn't provided the other four colors impressive toys of their own, or toys that blue just doesn't co-op (monastery mentor, drs, YPyro) it makes sense to take something away from blue. That's the crux.