My experience as a 4k player & charter for FFR/SM/Etterna/o!m* and someone who actively kept track of was being released on these games through the years (including stuff from the various subcommunities across the world) only shows me that mappers/charters are strongly biased towards making content that serves either themselves or the people they directly engage with in the community (forums, discord, etc). As people are more likely to engage in the community or consider charting after investing a considerable amount of time with the game, this usually means content for a skill level way above the general o!m population's skill level** and almost always single diffs.
I can compile concrete evidence from these games if needed but it's pretty clear the influx of new charts in both FFR and Etterna that would be playable for most of the o!m community is lacking in both quantity and quality. I mention the o!m community specifically because both FFR and Etterna nowadays have skewed demographics due to low influx of fresh beginners (lack of content is not the only reason but it's definitely a big one). On that note, I think even having the 'half-assed' lower level content some people mentioned as a potential negative from spread requirements is still better than the situation those games (FFR/Etterna) are in, not to mention I think any competent mapper who respects their own work would still try to make decent lower diffs even if it's out of obligation (if you disagree you should probably reread the sentence).
Returning to o!m specifically and together with what I mentioned about the general population's skill level**, I think there's strong evidence that difficulty spreads should still exist in some shape or form if ranking criteria wants to keep the general community best interests in mind. This is not to say I don't think there's an issue with good high-end content not being ranked (sorry for the triple negative), at least on o!m, but I think removing spread requirements is not the way to go about it.
Additional notes in *s* - I've been playing since 2009 and started doing it competitively + charting in 2014, focusing mainly on FFR & SM/Etterna where I'm a more known figure and where most of my content currently is (only ~30% of those are converted/uploaded to o!m). I also played various others rhythm games (especially VSRGs) other than the ones I mentioned, but the only notable one fueled exclusively by community created content that I'm more familiar with is BMS (LR2), where spreads for most songs already exist from the original event submissions.
** - In the osu website, if you hover at the success rate % of any difficulty, you can see the number of plays for that particular difficulty. If you pick almost any o!m mapset with a full spread where the highest difficulty is an insane or higher***, the highest number of plays is either a normal or hard (usually a low hard around 3*) and decays in both directions like a gaussian distribution. This suggests the community at large is or prefers to play things at that skill level and this is valid for newer maps as well. The number of plays should be highly correlated with the number of unique players but it'd be nice if someone on the dev side could provide that number to make a stronger case though.
*** - A similar and stronger argument could be made using data from multiple mapsets together, but it'd take way more work to make a reliable statistical argument that accounts for other variables such as song length, popularity, spread structure, etc. While I have no intention of making that kind of analysis, I'll note that the existence of a lot of mapsets where the top diff is lower than insane suggests that the numbers I used for the lower end in my original analysis underestimates the number of lower level players as those have more content available and their plays will be spread across more sets. This also goes against some claims that the game "doesn't need" more easy content or that players grow out fast from that difficulty range.