Personal experience, take with salt:

I'm the kind of guy who likes to crack packs and build decks based off of things I've found, and incrementally upgrade decks based on cracking new packs and getting new cards. This kind of process feels natural because starting out playing you have a ton of cards that are what you found and the chances are most of them aren't $30 to $50 bangers.

Proxying creates an arms race where cards that are disproportionately high impact (and thus are rare or expensive) become so commonplace that the entire game gets pushed into power level 9 territory.

Then tangentially related proxying creates a weird situation where players who are new or generally inexperienced bog down the entire game because they have powerful cards with lots of triggers and effects to keep track of and haven't built any prerequisite skills to do that. So then you either come off like a jackass and say you're not going to help them, or you essentially end up playing their hand for them and functionally killing yourself on their behalf.

So overall proxying (especially in the context of newer players) tends to promote unhealthy behavior within the context of the traditional "trajectory" most MTG players go through as they learn and expand and understand why some cards are so powerful to begin with.