Well, that's exactly what they've just done in the UK.
Prime has gone up from £50 to £80 and now includes 'Prime instant video' - which would be OK for free but contains pretty much nothing I'd choose to pay for and isn't available on my "ancient" 2010 Smart TV or by (newer) Panasonic BD player. So its effectively £30 extra for next-day delivery (which is what you get with Prime in the UK).
I'm ok until I'm up for renewal in October - then I'll have a hard think as to whether I'm going to need £70 worth of next day deliveries in a year. By then I'll have re-watched all the 'Buffy the Vampire Slayer' episodes I can stand
Thing is, its not a simple "#of amazon orders per year * average delivery cost" equation:
1. next day delivery is nice if you get it at a bargain price, but its not a must for everything.
2. I'm increasingly moving to eBooks, music and software downloads - and have been tending to give gift vouchers (which can be done all-elecronically) rather than physical goods as presents. That reduces the need for delivery.
3. Prime doesn't cover a lot of Amazon Marketplace stuff.
I hope Amazon's bean counters have done their sums right - as long as I'm paying for Prime, Amazon is going to be my first choice for online shopping. If I drop Prime I'll probably shop around more. Its not just the £50/year they'll lose.
They've dropped the ball in UK I think from the price increase. Our household has been incorporated through the love film buyout, and to say it's confusing is an understatement... We've dropped the instant movie streaming part of our subscription and the DVDs by post will go if they mess it up as it appears to be promising. I think I'll pass on this, and suspect I won't be the only one judging by the complaints on the Facebook page...