I think it'll take a while for any consequences arising from Sturgeon's resignation and the eventual SNP leader to filter through to voting intentions for the next elections. While I personally don't find the current crop of potential SNP leaders/First ministers to be particularly inspiring, there is still time for further entrants to the fray. There is also time after the appointment of a new FM for the SNP to sort themselves out and present themselves credibly again.
The SNP have built themselves a large and generally loyal electorate base, which to my eye is based on perceived competence, being the main locus for the independence vote and left of centre/progressive policies. However I think the situation is favourable for Labour - who have been quietly increasing their vote share from <19% (behind the Tories!) in the 2019 election to about 30% (September '22 onwards). The SNP are vulnerable to a renewed Labour who can credibly say that they are in with a big shot of being the party of the UK Government in the next election.
It's difficult to call one way or another, Labour have been increasing their projected vote share steadily for a while now, but the SNP independence bloc is likely to be large and will presumably vote SNP for as long as Scottish Labour remain committed to a unionist viewpoint. However, that said, Labour was once thought untouchable in Scotland, the SNP need to work hard to ensure that they don't make the same mistakes, especially if appetites for independence are lowered. Which I can see happening with a Labour victory in the GE. Many Scots have quite legitimate grievances with Westminster, which if Labour can sort, could render independence dead for a few generations.
