Challenge events are like eBay, only the top bidder gets the really rare and valuable stuff, that is why the 2- 10 prizes are not nearly as good as the #1 prize. If the 2- 10 prizes were almost as good, people wouldn’t waste time trying for #1. Someone selling something on eBay want the most money they can get from it, the Devs want the most people striving for #1. Improving the other challenge event rewards would water down the time, energy and money spent trying to get #1.
You can see this also in the event summons. The Devs could add event only 3* heroes and then just have a summons of ONLY event heroes. At least hearing people complain about a x10 summons getting five 3* Drunken sailor would be new. Instead the Devs deliberately make it hard to get the Guardian heroes by mixing them in with ALL the 4* and 5* heroes.
The tier completion rewards are like buy it now on eBay, not really and auction, more of a shop. Yes, I would like it if the completion rewards were on par with a rare quest, because that seems to be the amount of time, effort and money required to get them, but the Devs need a large difference to encourage people shooting for #1.
The “Save the Winter” even is almost exactly what you are describing. Perhaps we will one day have twelve monthly specials based on a holiday. The Devs are programming as fast as they can.
I think the biggest discontent comes from “wasting hero roster slots” saving 3* heroes for the challenge event and then the tier completion rewards being so poor.
Since day one on the forums I have been banging my drum trying to get more in game uses for 3* and 4* heroes, but no response from the Devs. Either a) they know and don’t have the developer cycles to do anything about it ( priorities ) or b) they eventually plan for 6* heroes in which case 3* heroes ( which are substantially weaker than 4* and 5* heroes due to only 3 tiers of ascension ) are going to eventual be the new 2* heroes, fodder and feeders. Only time will tell.