The success of Celtic and Rangers didn't ruin the Scottish League, it helped build it. What ruined it was how they used their power to squeeze the other clubs financially, taking a third of the TV money, etc. They were bullies, which is why so many people are enjoying Rangers' current plight.
The reason that Swansea are in the PL is history. The Welsh clubs didn't have their own league to join so joined some of the leagues in England. Cardiff, Swansea, Newport and Merthyr Town joined the Southern League (and possibly the Western League), Wrexham joined the Combination. At the time none of the leagues were national leagues. Even the Football League was only the North and the Midlands, with a single outpost in Woolwich. They never had an incentive to create their own national league (the current one is recent). So Welsh club football is intertwined with English football and has been from the start.
The Scottish clubs could have followed a similar path. They had played in the FA Cup, with Queens Park making the final a couple of times. But as the leagues began to form, the Scottish clubs set up their own league and stopped playing in the FA Cup. They chose to go their own way.
Clearly their plight is a mixture of both. The Portuguese and Dutch leagues have suffered a similar fate with the growth of the big european leagues. But these leagues have maintained their appeal. The big clubs haven't sucked the oxygen out of the league. Benfica, Ajax, etc are big clubs, way out of proportion to their leagues, but they haven't destroyed the competitiveness of the league through their greed.
If changes in football are to mean Scotland rejoining the English system, then it should be as a whole. Merge the Scottish leagues into the football pyramid. A lot of the objection to accepting Rangers and Celtic is because they are trying to escape a situation they helped create.