You’re right to argue that “once Scotland leaves, Labour will fight even harder to cling onto Wales than they did with Scotland”, but this presupposes that the Labour Party we know at present will still be in existence.
You’re also right to infer that Plaid Cymru is not the vehicle for change it would like us to believe it is.
If major change is to come in Wales then it is more likely to emerge from popular discontent with the political system in general, and growing contempt for professional politicians and metropolitan (and sub-metropolitan) elites, as was shown with Brexit, rather than from allegiances shifting within that rejected system.
The question then becomes, who or what is best positioned to take advantage?
At present, there is nothing. But in the period of uncertainty following Labour’s retreat to the left (resulting perhaps in the party splitting); and a growing realisation that Plaid Cymru is impotent; UKIP a circus overstaffed with clowns; and the Tories rejected by most Welsh due to something that might have happened in Tonypandy, or Llanelli, a hundred years ago, everything is up for grabs.
When a whole system is rejected there is little point in looking for leadership or change from within that system. Which is why Wales five years from now could be a noticeably different country, with changes effected, or influenced, by individuals, groups, sentiments even, that today we know little about or don’t fully comprehend.