STV were never the most loved of ITV franchises - for years, they were perceived (as they clearly still are by several posters to this thread) as dominated by a narrow, shortbread-tin version of Scotland, and the impression I get from those who lived through the 1960s/70s is that Grampian (which they have now absorbed, much to the chagrin of many in the north of the country) presented a paradoxically more forward-looking and cosmopolitan idea of what Scotland could be, despite serving a more conservative area (and before the great SNP breakthrough of 1974 still a quite widely Conservative area, despite the collapse of imperial unity). Nonetheless, this is clearly an important moment, especially considering that for a while STV (which intends to show at least two black and white films at peak time on Sunday night, it would appear!) seemed to be becoming essentially an ITV1 relay station. The conflict seems to anticipate, almost word for word and ideology for ideology, the standoff which is the most likely outcome of the next UK general election.
STV is clearly playing the long game. It's hedging its bets on Scotland feeling so disenfranchised by a Tory government at Westminster which has, in all likelihood, barely increased its current total of one Scottish seat that it opts to leave the union altogether. At that point, I suspect, it intends to merge with what is now BBC Scotland to form a new Scottish PSB (notice how it is using that Reithian aphorism, even if it is in a different order). As is mentioned in the Digital Spy thread, it would no doubt be competing with a wholly deregulated ITV1 on its doorstep, and plenty of other commercial broadcasters which would continue to operate in Scotland (cultural autarky is no longer an SNP aim, really), but it would have an audience, and probably a greater one than most DS posters are willing to admit.
The question is: would that be enough? Scotland only sustains its higher investment in public services and more "public" social ethos, epitomised in STV's quasi-Reithian language, because of English money. Likewise, STV can only do what it's doing - aiming to capitalise on a growing sense of disconnection from the UK by moving away from being the ITV1 relay station it had seemed destined to become - because it gets enough viewers for Coronation Street, Emmerdale and The X-Factor to sustain it, at least for the time being. Scotland's only hope of sustaining its stronger public sector, the strengthening of which would be precisely why many Scots would want to break ties with neoliberal Westminster, would be EU funding, and lots of it. To be fair, I think Scotland might succeed in that aim, especially if (as would clearly be its role model) it takes the same proactive approach to the EU as Ireland has, rather than whinging a la Westminster. The question for a post-switchover STV would be: could it find a similar key to the door?