You are only looking at one side of the proverbial coin - and only at the coin that would ultimately land in your pocket.

Bank transfers have waned in popularity steadily for years - when I first joined eBay (2008), they were pretty popular, I used it to pay for all of my purchases (and I, luckily received all purchases except the first, but the seller refunded), then when I started selling in earnest, up to half the sales I made on a weekend would be paid by bank deposit (my initial foray was to list a bunch of items every fortnight).

I eventually opened a store and built it up to become relatively successful, and I kept bank deposit as a payment option for multiple reasons (towards the end, it was purely so I could get checkout to remain functional, lol). It went from being used several times a week, to several times a month, and eventually to maybe once or twice a year out of thousands of sales. I also went from preferring it, to dreading it. (So many more problems, like buyers shortchanging and still expecting the item to be sent, or not paying at all and pretending they did, and still expecting the item to be sent, or getting the account number wrong and never getting their money back, checking my account balance constantly so I could post the item as quickly as possible, just a PITA all around. though I know a couple of these complaints can be eliminated with current technologies, like Pay IDs; I also now get notifications on my phone whenever a deposit is received into my account). 

From the buyers side however, those complaints are losing money to a scammer and having zero recourse. Sometimes people have paid thousands of dollars thinking they actually had eBay protection, only to find out very swiftly that no, they don't.

This is a marketplace, and that means in order for it to thrive, consumers have to have confidence in the site they're buying from. It doesn't matter that they had a choice, what matters is the story they tell about their eBay experience. 

I do not work for ebay, I use ebay to make a living, and as much as it might invite the conspiracy theorists to do the thing they always do, I will gladly give up the extra $$  (or cents), that I made from not paying PayPal fees on bank transfers for all the issues it caused, and never having another buyer lose their money to an out and out scammer, so they know they can all purchase here safely, which will = more sales overall. 

Again, I will never say there are no downsides to the managed payments system, I still have concerns particularly for sellers, but in this specific case I do think the overall benefit to sellers (in general) is being overlooked here.