ECW (the post ‘Eastern’ years) was basically the foundation for what would become the Attitude Era, so without it pro wrestling may have taken much longer to leave the cartooney character world.

Deeper/edgier/mature storylines became more common place due to ECW’s influence, both of which Bishoff and McMahon began copying, but which a lot of people wanted as an alternative to the storylines of the big guys.

Heyman was a sack of shit who was an abysmal businessman but creatively (especially for the time) the man was a damned genius. He knew how to get every last drop out of the product they had (“accentuate the positives, hide the negatives!”) and put on a product nowhere near the production of WCW/ECW but was still good enough to get a tv deal (for a short time).

Heyman had a focus on shifting Eastern Championship Wrestling into an extreme niche to draw a fanbase WCW/WWF-E weren’t taking advantage of, but also didn’t let go of what got them there: great wrestling. As such ECW became the first US promotion (or first big name US one at least) that would book people like Rey Mysterio, Psicosis, Eddie Guerrero, etc. Which of course lead to so many people being poached by WWF/WCW.

Compounding on that, ECW was similar to ROH/PWG in how you look now-a-days and so many people in WWE/AEW came up through them, a lotta big wrestlers came up through ECW or helped them with giving them a place to form their characters (like Stone Cold, the basis of which was formed in ECW before he went to WWF as the ‘ringmaster’).

ECW’s fanbase was as toxic as WCW’s, WWE’s, and AEW’s. Some loud mouth jackasses, and a whole fuckload of normal people who just didn’t like WWE/WCW and wanted to see harder wrestling. Now-a-days most of these same people would be fans of GCW and GCW fans are some of the most accepting, least judgemental motherfuckers I’ve ever seen (especially given the huge POC and LGBTQ+ wrestlers/shows they book) and when someone starts shit they usually get knocked the fuck out.