How hard is it for Plat players to reclimb to plat consistently with no duo?

The skill level of plat has gone down this season.

Depends on what you're playing, as always. As a Mid/Support/Jung Player, I tend to try to grab Adc duos, and turns out they're usually hit or miss. Adcs don't generally carry (unless they're Ashe or Jhin) without a competent team, so even if they go 15/3 every game, they still haven't carried.

If your climb is based on playing Nocturne or Kha Zix, the rate of climb is MUCH faster, more reliable, and overall toxic for the people you are playing with.

Also, it depends what season you were Plat.

Plat I Season 4 is about Gold III in Skill level now. Plat I in Season 8 Is about Plat III now. Plat I in Season I is probably Silver by now.

The ability to solo/duo carry has dramatically decreased over time, because there's so much early gold people can grab, and the games generally spiral out of control faster and harder because of that. If you aren't playing the role at the nexus of all that gold, you probably can't carry as well "playing with respect" and trying to CS 8/9 a minute, and waiting until later to accumulate gold. In reality, Plate and kill hunters are carrying far harder than respect players right now. If you play the respect game, you generally lose about 1k gold in income, on face + whatever kills you could have had.

In Contrast, High elo players used to play nothing but the "respect" game, so most are willing to give up the plates in the favor of a cautious game, with the exception of sometimes Top, which can usually push more reliably than bot/mid, without getting in too much risk/feeding danger, if you are winning lane.