But you're still talking about the effect of being attacked by multiple players, not the effect on the game of (you, one player) buying an attack. Sure, the tempo is faster in multiplayer, but that is true whether you buy the junker or not, if everyone else buys it.
Okay, not junking the others will probably slow them down a little less. (Obviously between 3-player and 4-player there's also a big difference, in 4 player it's pretty marginal whether the others get junked by two or three players.) And that could lead to you getting junked more than otherwise, but it also leads to the others junking each other more than otherwise, offsetting the slowing down. So I don't think you get junked much faster if you don't buy the attack. The main difference will be when the junk runs out. At that point, after that shuffle, will be the first time you're deck is really worse in terms of junk, and now the fraction of junk is exactly what matters. (But now your deck should be better in other things.) This is not accounting for trashing of course.
In any case, the initial claim was that attacks are less good in multiplayer, and that is obviously true. With Militia it's very obvious; you're literally not attacking all the players, while in 2-player you always are. With junkers, even it were true (and in some kingdoms it's more true than in others of course) that buying the attack is very good and important, it's still less so than in 2-player. I don't see how anybody can disagree with that. (This is ignoring attacks that you actually benefit from and that target players, like Pirate Ship and Thief.)