A 5th level Ranger spell isn't more or less powerful than a 5th level druid spell, by and large. They still use the same base damage calculations and so on. The difference is, paladins and rangers don't get 6th+ level spells, but, instead get other class effects instead.
The "Nth-level" label is meaningless. It's just a label*. The system effectively
does vary spell level by class -- it just disguises the fact by saying that druid get "5th-level" spells at 9th level but rangers get "5th-level" spells at 17th level. They could have said instead that all classes get "5th-level" spells at 9th level and "9th-level" spells at 17th level, but the ranger's "5th-level" spells are "3rd-level" spells for the druid and its "9th-level" spells are "5th-level" spells for the druid. It would have been more confusing for other purposes, but it would be more accurate for the purpose we're discussing here, of class balance. The spells that a ranger gets at 9th level are less powerful than the spells a druid gets at 9th level, and the spells that a ranger gets at 17th level are less powerful than the spells a druid gets at 17th level. Comparing a druid's 9th-level spells to a ranger's 17th-level spells is comparing apples and oranges, even though they both happen to have the label of "5th-level" spells.
So much for half-casters versus full casters. Now, even between two different full casters, there are power differences. You say that spells of equivalent level do equivalent damage, but that's simply not true. Clerics get
flame strike at "5th level" (9th real level). Wizards get
fireball at "3rd level" (5th real level).
Fireball does the same damage as
flame strike, in a wider area, at a lower level. This is because wizards are generally accepted as being better blasters than clerics; more of the power budget in their spell list is assigned to good blasting spells. Clerics, on the other hand, get very good healing, which wizards can do poorly at best.
So if a psionic class got a "3rd-level" (5th real level) power that was equivalent to or better than the wizard "5th-level" (9th real level) spell
telekinesis, this would not necessarily be a balance problem any more than
fireball vs.
flame strike is a balance problem. It
could still be a balance problem, of course, but it would need to be evaluated as such holistically, taking into account the class' power budget as a whole, not picking out single abilities in isolation. To take the hyperbolic example, if that were literally all the psionic class could do, then it would clearly be weaker than the wizard, notwithstanding that the wizard can't do that specific thing until a higher level.
*Except for bards' Magical Secrets.