I have a lot of experience, in each edition, with both sides of the argument. I have played in games, in each edition, where a dead PC means starting again at first level regardless of the level of the rest of the party. I've played in games where DMs give massive role playing experience bonuses, most often to the really cute girl in the group that says very little but shows a lot of her "character" (character is a measure of skin, right?), and I've played in games where PCs are ALWAYS 7th level. In my life, I've probably played in multiple sessions with over 200 different people, and done one shots with several hundred more people. Been there, done that. About 40% of those games used milestone experience and the majority of the rest used experience advancement, at least in theory. My experiences over that time just do not match up with people preferring to have lower level PCs because they can't make every game due to real life responsibilities, or with PCs being several levels behind being no big deal.
6 levels behind? Forget 1 vs 7. Fire giant is CR 9. A 3rd level PC has about 20 to 35 hps. The Giant deals 28 to 29 damage on a swing. 6 levels behind means going down in about one blow - maybe you're 'lucky' and the first blow takes you down to 1 or 2 hps where the second blow may be a single hit negative hp total kill. A CR 13 Nalfeshnee might need all three of the multi-attack attacks to take down a 7th level PC with 40 to 65 hps. Maybe. Marilith of CR 16 versus a level 9 PC? Tail grab, restrain, then slice and dice with advantage for 6 more attacks? Heaven forbid the PCs face off with a monster with a CR *higher* than the toughest PC in the encounter - better to bore them with foes that are not a challenge to them, right?
A lot of the DMs I know that have been players in my games move to milestone advancement eventually - some right away. It is so much easier on the DM. It aligns advancement with a good climax - a place where rewards feel earned. People feel less over/under powered when facing challenges - and that really does sit better with pretty much all the players I know.
Green Eggs and Ham, people. Green Eggs and Ham.