I dont think it will happen, but I do think that at some point in the next few years Rangers and Celtic will breakaway from their league along with 5 or 6 English teams from the Prem and create a couple of 'Euro League' divisions.
I've seen a few ex pro's and managers etc talking about it recently, maybe they have the insider knowledge. Not sure how good it would be for the rest of us. Like you say, without the Old Firm the Scottish League would fall apart to sunday league level. Down in England wouldn't be much better either ... all the cash would be going into the Euro league and the leave the rest of us struggling with buying players and scrapping for the odd place into Europe.
I dont think there would be too many places available either. If the top clubs were to break away, I'd think that the likes of Utd and Real would be given/would want permanent 'seats' in the league or something similar.
Anyway though, IF Rangers and Celtic ever did join the English League, I think the only way it would work is that they start at the bottom - League 2, they cant be expected to go lower than that.
At the moment the two would be average Championship teams but within a few years they have the potential to be massive and actually end up dominating. Huge fan bases, histories, stadiums plus the extra revenue the Prem brings in - the first Scottish champions of England? Sounds scarry but maybe thats the real reason it has never gone through before ...