Your arguments make sense, but I still don't agree with them. It's like me trying to say that Latvian bloke who scores 40+ league goals every season is as successful as Messi/Ronaldo. Or Jordan Rhodes is as successful as van Persie. Relativity is important. But more importantly you're ignoring European achievements, which is the most important factor of all. Madrid and Milan who I mentioned have 16 European Cups between them. Barcelona have 4 European Cups and 4 Cup Winners Cups to go with their domestic success.
I'm not saying Rangers are a better team than Barcelona, it would be very hard to be with the way football is, there never really has been a level playing field, and although it used to be more level than it is now, Rangers and Celtic have always punched above their weight and still do, just not on as big a scale, and that's only because the financial disparity between countries and leagues is getting wider. Celtic won the European Cup, and got to the UEFA Cup final in the 2000's.
We won the cup winners cup, gwe got to the semi-final in the first year the European Cup became the Champions League, we lost to Marseille in the semis and it was later found out they match fixed, we accumulated a lot of debts in the 1990's and early 2000's to try and keep up with the way finances in other leagues was chaning, and it nearly worked, that season and it did work most years domestically. And we got the UEFA cup Final in in 2008, beating teams from all the 'major leagues' on the way to it.
Personal achievements like the Latvian striker thing, aren't the same, my argument isn't saying a Latvian striker paying in a part-time league scored 60 goals in all competitions so he is as good as Messi, that Latvian striker probably wouldn't score 60 goals a season if he moved to Barcelona, but that doesn't mean his achievements are rubbish beacause he wasn't playing in a league where, if you get relegated you will get a parachute payment higher than most teams in other countries would make in a year, a finishing in the top half of their league,
I get what you're saying and there are different ways of looking at it, but judging it, the team with the most trophies are the most successful, just as the guy with the most medals is meant to be 'the best of all time', and we have more trophies than any other football club in the world.
Even though we would find it difficult to play in the Premier League or La Liga, the SPL is a good standard for us just as La Liga is a challenge for Barcelona, so the achievements are just as good, and in this case better, because every team, athlete, etc. have their level.
And the fact that Rangers and Celtic can lure good players, and when we're in division 3 and been punished really badly, is amazing for the clubs, and the reputation they have although financially football becomes more unbalanced, and players sometimes have to take lower wages to play for us.