You need to get 50 blues from gachas every month in order to buy all 10 fates. 50 blues is about 55-60 total pulls.

In 1 patch (42 days) you get about 65 event fates and 10-15 blue fates (5 for dust every month, 5 from bp). So on average it'll be maybe 14 standard every month (+ character ascensions) and 40 event gachas

Standard gachas are used instantly. That means you still need to make about 47 pulls on the event banner every month to get all 10 free fates. But if you only get 40 on average, then you still need to somehow get 7 more from other sources (exploration mostly. Clearing abyss with 33-36 stars helps too).

So no matter what, as f2p, you are always very tight on dust and can barely afford all 10 fates (sometimes you might even have to skip a couple standard gachas). Spending 85 on Dust of Azoth will most likely cost you a gacha.

And now imagine you want to skip a banner, now you have to choose between risking a pity reset by getting a 5 star you don't want just to get enough stardust for all fates, or missing out