I'm not encouraging anything. We're talking about retail, not classic or private servers. The only reason I mentioned those is that those are the available options for people who want to do legacy content that's scaled properly. Can you play with people on classic/private servers from retail? I know it's a redundant question, but that's because we're talking about retail and not those. You can argue about splitting the playerbase across various version of WoW, but what I'm talking about is splitting up the RETAIL community. If you make every dungeon/raid relevant at max level through some scaling system, you go from having a pool of 10-12~ dungeons and 1 raid at the start of an expansion, to 100+ dungeon/raids right off the bat. In this case, more isn't better, especially recycled content that most of the community has already seen.
By having everyone on the same playing field and making the current expansions content relevant, it means everyone learns together about the new dungeons/raids. Everyone begins to learn the mechanics and how certain mechanics work. It's new for everyone and we'll all have to work together to figure out the solution. Old content doesn't require any thinking, it just requires someone who knows the fight to explain it to the few who don't. There's no thinking or puzzle solving, it's just taking a swim through nostalgia. That's not the main issue though, the main issue is splitting up the community. If you look at raid finder for pugs, most of the raids you'll see are for the current raid and there'll typically be a few for older raids in the same expansion. If we go from having a pool of 4 raids to 30+ raids, don't you think that would saturate the raid finder? Now if someone wants to find a raid for the current raid, there might be less groups to join, therefore leading to a lower chance of getting into a raid because we now have a few naxx, ulduar, ICC, Firelands, AQ40, Throne of Thunder, and HFC raids all going on at the same time. I doubt that'd be the case when a brand new raid comes out, but 4-5 months down the road? The playerbase begins splitting and people are raiding older raids rather than the latest raid that came out. There's no relevancy to do it since gear of equal value is dropping from older raids.
The whole concept sounds good on initial thought and it's something I think blizz might do if the game is still around in 10 more years, but for the sake of maintaining relevancy of an expansion, resurrecting legacy content and rescaling it is a bad move.