A good sticky jump does 60-70 self-damage, and landing on any platform that isn't significantly above your head will add enough fall damage that you won't comfortably be able to do it twice. Of course they can do it more than twice by picking up health. So what?
Have you played any map that isn't dustbowl or goldrush? First off, nobody is outside the spawn? Second, how stupid does a team have to be that one demoman can hold them off? Third, most maps (especially the ones that aren't from 2007) have more than one spawn exit. Fourth, respawns are not flat 20-second rates. One group can spawn 2 seconds after another group has left. Demoman doesn't have a chance against that. Any entire team that gets repeatedly spawn camped by the same demoman in his same life, who doesn't even have a medic, especially on any map with more than one spawn exit, deserves to be killed over and over again.
I don't even know what maps you're talking about. There aren't many situations where an engineer is around, yet a demoman should be able to sticky jump to spawn effortlessly. It just shouldn't be happening. It's the team and the engineer's fault. If you think a demoman destroying teleporters is even the slightest bit of a pressing balance issue, you need to play this game much more.