I feel like the best approach would be to instead change the order of when skills are obtained, including making more weaker versions of higher level skills; so that the lower levels can at least not be quite as dull in regards to class kit. Or at the very least having the bare basics of combat available by the time you reach Sastasha. Why is it that we can't do our most basic 123 combo and let every job have a spammable AoE by level 18?

It would be very nice for a job to at least feel 'put together' by level 50 or so with the core mechanics intact, with flourishes, nuances, and flashiness left for later levels. Why can't I have a weaker magic phase on my level 50 Paladin? Why can't I have Fire IV also at level 50 on my Black Mage? A lot of jobs would feel better at lower levels if they had something resembling their core rotation earlier.

As for the idea of syncing not removing skills at all, there's too many issues with it. Firstly, some abilities just don't sync down well at all. A tank invuln for example is going to make you invulnerable regardless, you can't make something only 'slightly invulnerable', because it would still be invulnerable. Secondly, it doesn't account for player skill differences, someone synced down with a full kit that's still trying to piece together how to best use their buttons will end up doing less damage with more buttons compared to the person next to them only pressing their 123 combo because that's all they've got. It makes progression feel awful like that when you have to put in thrice the work for the same output.

I could very well sing a different tune if such a thing got implemented well, but I think the best approach is to introduce core combat and class skills earlier than what we have now rather than letting a level 90 tank invuln their way through early dungeons while a new tank gets left in the dust.