i don't think so... there are some great decks that are still very winning without putting any legendaries in the deck. the example you used is great and all, but you're completely overlooking a few things; synergy being the most important. i'm not going to lie and say i don't use legendaries. that said, i use them sparingly depending on the synergy. lets say i have a captain cucumber in lane 1, bananasaurus in lane two, and two party thymes on the board. i play plant food on captain cucumber, that draws a legendary for me, buffs banana... then each party thyme also draws a card and buffs banana. thats a good use of the legendaries in that deck, but it also doesn't mean that party thyme and plant food weren't equally powerful in that situation. it also would have been zero help to have a doubled mint on the board.
you pick out one or two legendaries that work well together, then you build your deck around that. you don't just throw a bunch of legendary cards into the deck and hope that they can work together. also, look at the cost of that combo, and what's involved in making it work. the earliest you'll get a decent return from that combo is on turn 5. during that time, they could play a weed spray and destroy your well designed combo... they could play smelly zombie in front of banana... they could play bonus buckethead... playing those legendaries certainly isn't a gimme, and certainly isn't overpowered considering how easily it can be shut down by playing cheap cards.
yes, legendaries are often gamechanging cards. they should be. they're hard to get; and while they may have powerful effects, they often require patience and skill to play correctly to get them to the overpowering point. they can't just be thrown out there and expected to win the game. it's how you build the rest of your deck that makes the legendaries strong, and filling your deck with all legendaries isn't that build.
