Based on 2017 here's the maximum that Paid can get if all of the Green/LD voters switch

Pontypridd - 6,000 (3rd)
Llanelli - 8,000 (3rd)
Dwyfor Meirionydd - 14,000 (1st)
Arfon - 12,000 (1st)
Ynys Mon - 11,000 (2nd)
Carmarthen East and Dinefwr - 17,000 (1st)
Caerphilly - 7,000 (3rd)

The total votes of Lib Dems is fewer than 1,000 votes in all but one of these constituencies. The only place it would impact is Ynys Mon where they'd move from 3rd to 2nd and the only place where the Lib Dems stepping aside would have a genuine effect, Ceredigiong, sees both of them going head to head! Plaid are such a joke at times, what I'd give for us to have an SNP style nationalist force in Wales rather than this lot