Playing with numbers:
this is boss HP for dungeons as we level. First is the current numbers, second is the numbers scaled by a flat value. Third is a squish based on a scaling value (in this case, HP/lvl), last is scaling and flat value (HP/lvl/10). I have displayed this twice: first uses a numeric scale, second is a logarithmic scale so you can better see the changes.
Keep in mind that boss HP at a set level is dependant on encounter mechanics: if there are adds, or times when the players don't attack the boss, typically the boss has less HP - this accounts somewhat for why I don't get smooth lines. Also note that I threw this together in short time, and am not a mathematician, so it's certainly not a viable solution here and it wouldn't surprise me when smarter people then I blow this all out of the water due to something I haven't thought of.
Anywho, the point I am trying to make here is that it's possible to apply a variable scaling factor that will squish the game numbers by a significant factor, put still maintain a relative power gain at each level. Blizzard can choose to massage the data even further to make the lines curve more neatly, if they wanna.
TL;DR: a squish can still allow soloing of old content.