This line of chatter has been going around UK-centric online discussions and it's such a braindead line of thinking. Marginal pricing is used in a gazillion countries and is tried and tested.

You do marginal pricing because otherwise providers will simply bid what they think the crossover price will be - marginal prices lower prices in the long run because providers try to improve their margins and lower their bids.

Also renewables bid on the spot and day ahead markets really low because their cost is almost entirely capex (and also most are under CfD anyway so may as well bid 0) - the spread isn't nearly as good as the data would imply. If you didn't use marginal pricing you'd have them bidding higher to cover their capex.