The effect of overcapacity is null or negative price, which has the property to make more storage viable (who cares if it only gives back 25% if the input is free or very cheap), so I'd say intermittent sources overcapacity is an enabler of on grid storage.
E.g. today in Germany you can buy MWh at 14€ at 13:00 and sell it back at 180€ at 18:00. I didn't look all of Europe but it looked like the biggest spread today... You can make money with crappy storage under those conditions...
This is precisely why intermittent sources aren't viable without a breakthrough in energy storage. Existing storage mechanisms aren't capable of delivering at the tens to hundreds of terawatt hour scale required to make intermittent sources viable.
Remember, 66.8 TWh of electricity is used daily. Intermittent sources don't just experience daily fluctuations, but seasonal fluctuations lasting days or weeks. Even 12 hours of storage would still leave us with periods of insufficient production multiple orders of magnitude more frequent than the status quo: https://www.nature.com/articles/s41467-021-26355-z
E.g. today in Germany you can buy MWh at 14€ at 13:00 and sell it back at 180€ at 18:00. I didn't look all of Europe but it looked like the biggest spread today... You can make money with crappy storage under those conditions...