I just need to point out something first and foremost: for an overwhelming amount of people, they are both PvE game modes. Nothing more, nothing less. This is why folks who participate in any PvE in WoW is likely to also participate in other forms of PvE. Delves being an exception due to it being completely soloable.
So… the first thing would be to acknowledge that there’s no “sides” here. Just folks who enjoy PvE and then the oddballs who decidedly don’t like some forms of PvE, with many of those people engaging in some egregious “No true scotsman” fallacies on these forums. All of this meaning that your question is largely defunct and doesn’t have a proper answer, except the one…
This is the wrong framing of the question. None of them are harder than the others in a void. You cannot compare raiding to M+ the way you are attempting to do. For the same reason why a Delve cannot be compared to either raiding or M+ for that matter.
Delves are part of the conversation as they demonstrate the limits of what an endgame soloable activity is, and also how such an activity affects the overall pacing of other areas of the game as well. But M+ and raiding are both group content, designed for different sized groups.
If we opt for a “Difficulty per group mechanics” raiding will always have the most difficult mechanics because it is has the most amount of people. If we opt for a mathematical and performance style view of what’s most difficult, M+ scales infinitely whereas Mythic raids doesn’t.
So the answer is that they are differently difficult because they are designed with different elements in mind. You cannot compare the two of 'em the way you are doing. So the answer is … ask a better question, or frame the question in such a way where it is possible to answer it.
This is for an example such a question.
Are the rewards proportional to the content?
Here one can debate quite lively and I’m firmly on the side that Delves are scaled appropriately. Heroic raids aren’t massively complicated and anyone who want to can get achievements like AOTC if you put in the effort. So that means that soloable gear providing heroic gear? Sure. Now whether it should be obtainable as a drop or via the vault… The vault would likely be better for the game but having an additional weekly option for a piece of heroic gear would likely make the gearing feel better.
Delves need more work in getting the pacing right but overall, it is fine. And on the same token of stuff that is fine, M+ gearing is also fine. It cannot be a source for Mythic gear that’s droppable outside of the vault as that would be frankly insane, but as it is a mode that can be scaled mathematically up to infinite, that certainly deserves to have access to the highest possible gear in the game. Not to the degree where said gear can drop from the activity itself, that’s reserved to raids, but from the vault? Yeah sure.
So… yeah. There’s not really that much of a debate when framed as “either or” as you have, but if one want to discuss certain aspects such as gearing and how it influences each others, sure.
But from that angle Delves, M+, and raids seem to be fine. Mythic raids might need an increase in gear but I haven’t heard of a convincing argument for that, so that’s more of a potential maybe rather than a definite. So overall, it is fine and works exceedingly well as it is right now.