that's an interesting angle thinking that flips will need to be replaced with grabs. the only realistic place that a grab could even replace a flip is down stairs/gaps - which is horrendous. otherwise, are you saying that people should be doing grabs into and out manuals and ledges and just generally on flat in the middle of lines?
i think how and what skateboarding is, is already established. it's already been refined and progression now is really just higher, longer, better style and to that point skateboarding could kind of be passed technical progression {i know that it isn't - i guess this is just my opinion}.
it's just an established thing that doesn't even need progression in the act. like when you just enjoy a particular genre of music for a weak example, it doesn't actually have to be progressively more, it's just the same shit done by different people and you can just keep appreciating because you dig it. not all bands are as good as each other, but the better ones aren't always differentiated by more complex chords and intense polyrhythms.
people already know that double flips or 540 flips and beyond don't make things better. i think progression for skateboarding is more in how and what the culture is more than anything else at the moment.