The problems are many-fold.
Among others that Magic is a trading card game and and any implicit expectations for MTGPQ to be the same are, at this point, very far from true. In paper magic you can actually get all cards, just by throwing enough money at the problem, good luck with that in MTGPQ. Paper magic partly solves the saturation effect of obtaining “all” cards by having various special editions etc. that increase collectibility and give the hardcore collectors something to do. MTGPQ has tried something similar with the introduction of ‘polished-turd’ masterpieces with an argument along the line of ‘it will look really cool in your collection!’. Except you can not show off your collection to anybody. And except that in paper magic if you are not really at a stage where you are interested in collecting odd cards, but just need a playset of whatever to complete your deck, you can trade in that ‘cool-but-not-neccesary’ card and everybody wins. This is the reason it works.
In the end, cards have value because the game is good (eg. they are fun to play with) and because they are powerful (they give a competitive edge). This also holds for MTGPQ, this is why people felt that paying 30$ for Olivia was a sweet deal - it gave a competive edge. When you decrease rewards in all events, you thus devalue all cards. It does not really matter if you top 5, 25 or 50, the payout is very similar - and pretty bad. Then there is the aspect of fun, as has been touched on - getting new cards is fun, even if it’s not a super-tier card, some new cards open up a host of interesting - fun - decks. I fully agree that this is partly what keeps events ‘fresh’, if you’re always playing the same objectives, with the same cards, it does get pretty tedious. The game is at its best - for me - when you can brew new decks for certain objectives, refine and tweak as you go. At some point this needs to be stirred up, either by new event (or event objectives) or by getting new toys (cards). With decreased crystal and mythic payout, the latter is certainly not happening as often as it used to.
I do think that prizes may have been a bit generous to the top before. You actually have a very good tool for tailoring prizes now via mana jewels. Mana jewels actually has the potential to bridge the huge gap between a rare and a mythic, by giving out some fraction of a mythic. It might be worth considering that a very low fraction of a mythic for even a high placement is not very encouraging. Particularly when you can get large amounts of mana jewels for $$, that kind of just reinforces the point ‘why pay to win, when what I win is worthless?’
Then there is the new player experience, with the decreased payout and drop rates as they are, how do you expect somebody to remotely catch up to the top players? You need a solid selection of cards from 3 sets (arguably, mostly SOI/EMN) and as evidenced by this thread, you can hardly get there.
What is the harm in people being able to collect most all cards? Give the whales something else to splurge on and chase. In the end, you need a sea and some small fish for the whales to swim along. At the current rate, you’re drying out the ocean at an alarming rate.