Why would you eliminate gear progression?
Let boss abilities do %hp damage. Defensive stats on gear lower incoming damage. AP and SP remain on gear along with secondary stats
1) Keeps the challenge.
2) Raiders with better gear are able to finish an encounter with a higher success rate and/or less time.
In my opinion, raids consist of epic level encounters which require multiple heroes working together to defeat them. Thus there should be mechanics in place that don't allow or make it extremely hard to solo old content. There's a reason they're called 10-25 man raids, isn't there?