There is a HUGE increase in difficulty when you hit D3, since D3 plays with D3-Pred, while D4 plays with Plat – D4. There are no "Diamond" ranked queues. D4 feels like a Plat lobby, and then D3 is just straight preds. The jump in difficulty from D4 to D3 is extreme. I imagine most of the diamond players get discouraged and then just stop playing ranked.
With higher player counts due to crossplay, it would be so much better to have a Diamond queue. Plat 2 through D3 should play together. This acts as an entry point for high-plat players to prove they are good enough for diamond, and also forces low-diamond players to play against better competition rather than farming low-plats. Most low-plat players don’t stand a chance against a Diamond player 1v1 and they shouldn’t be in the same lobby. Once you hit high-diamond (D2), you should play with D2 through Pred.
The split is also way too short, which is why pred lobbies shouldn't start until D2. The first two weeks of the split are spent ranking back up to your previous rank before the reset. One of our squad-mates couldn't play for two weeks so we ended this last split at D1, about 500 RP away from Master on console. So now I literally get no additional reward for grinding through pred lobbies all the way to D1. My only reward is that now I get reset to plat 3 and have to play in sweaty plat lobbies full of preds for a week and then have to go through D3/D2/D1 AGAIN. Pred lobbies really should start at D2, so that Master is more accessible if you have the skill but don't have the time. Top 500 Pred remains exclusive to only the most die-hard of players.