Well if people are paying for the BC games, individually or through a sub, SNY can make it worth it, assuming there's enough interest. I also agree SNY should push to make sure all possible PS BC titles are available asap. Third party is another story since it's not as simple.
There would be some sub downgrades, no doubt about it, but my question would be how many who aren't paying for Plus, who have a PS3, PS4, or PS5 right now, would get the PS Pass since it would only be $30? Some won't sub no matter what, but how many would for a year of online, for only half the price of a game? How many will upgrade from Plus to Premium? Lastly, how many newcomers will enter the PS ecosystem who wouldn't have otherwise, due to the new PS Pass or PS Premium level being offered? Can this at least offset Plus as is now, and/or can it grow the service user base?
If they're going to fairly closely compete with GP content, I'd personally like to see a layout like this, which would actually be 4 tiers.
$29 - PS Pass - (basic online gaming access)
$69 - PS Pass Plus - (PS Plus as it exists with it's online access, free games, and discounts)
$149 - PS Pass Premium - (PS Plus, PS Now, day 1 exclusive new IP, all BC games made available, previews and early access, better discounts)
$249 - PS Pass Platinum - (PS Plus, PS Now, all day 1 exclusives, all BC games made available, previews and early access, VIP discounts, etc)
SNY could not have the $30 tier, leaving 3 tiers total. Though that base tier (Plus) or whatever it were called, should remain $60 or drop to $50.
Plus at $70 would simply require either a few more free games per year, or just higher quality overall compared to prior. (only with $30 base option)
With GP ultimate being $180, I don't think SNY could go higher than $250. With their proven track record of first party exclusives, at $70 each, with usually 2-3 dropping per year, $250 should be worth all the new exclusives, plus VIP discounts.