If you think your legendary is forever future-proof, you're still buying into their (false) pre-release hype.
Honestly, I don't see a level cap coming, mostly because 10 more trait points will make some ridiculous builds totally possible. For example, with 80 trait points, a guardian would be able to pick up Right Handed Strength, Empowering Might, and Altruistic Healing. Combinations like that would be borderline broken, and the right builds could potentially trivialize what difficult content is left in the game.
Of course, ANet could opt to make more difficult content, but then we're treading into more grind/treadmill territory, especially if this new content would be accompanied by new gear.
Not to mention, bumping the max level would make all of the level 80 gear we have obsolete. I can't imagine many players would be very happy with having to grind out another set of armor and weapons for each level 80 character, some characters having more than one suit of gear.
As for legendaries being top tier, I think they'll hold to their word that legendaries will be equivalent in quality to any ascended (or potential higher tier) equipment. The problem I have is really that ascended shouldn't even be in place. At least, not with higher stats.
I would've been completely fine with ascended equipment if it were only for fractals, and the only difference between it and exotic was having an infusion slot. But even the slight increase in stats from exotic to ascended is indicative of ANet's change of stance. It took a whole 3 months post launch for them to completely back track on their manifesto. After that, I don't have much faith in them or any of their promises.