Well, really now, if the game only gets harder because you need to roll higher on your dice, something's lost on the designers.Yes, those strange, strange people who believe the game should get harder as you go up in levels instead of hitting Vecna on a 8.
Vecna shouldn't be hard to beat because you need to roll high to even hit him, but because of what he can do. Otherwise he's just a nasty lich with 3 points more on his attacks and defenses.
It's not like you can't make a game harder or more challenging and keep the players enjoying it and succeeding on their actions frequently enough to keep them from getting frustrated or making back-up solutions for their emergency plans or start min/max'ing just to have a decent chance at hitting.
Any solo would be perfectly fine if the players could hit it on a, say, 10 and it'd have the attack and defense powers to last against them. Give it immunity to the worst disablers (maybe allowing for one of them to work so the players can discover that weakness and benefit from that discovery), some defensive powers like the ability to roll saves before effects kick in or the ability to treat some conditions differently (merely being immobilized and granting CA instead of being fully stunned, for instance), and some nasty offensive powers and the ability to get more actions in a turn.
It's always more fun to see your power have some effect against the BBEG (even if the BBEG recovers relatively quickly) than to hear that you missed again, despite rolling a 14...