i wonder who the first will be to only offer yearly subscriptions.

I don't agree thats likely to happen. Consumers are price sensitive. The linear demand curve has price elasticity at a specific point. This is where a service can obtain the maximum number of subscribers at a specific price that will net the service the most money. Raise the price any more and the service earns less money even with the higher rate. Lower the price, and the service can pick up more subscribers, but the overall loss of profit will leave money on the table.

The likely $99+ annual subscription rate with no monthly option will drive so many customers away, the streaming service would die.

i guess you could say amazon already does.

Not only can you get Prime subscription on a monthly basis, Amazon even offers a Prime-video only monthly sub with zero shipping benefits for even less.