Where did you see 15x as the multiplier?

For US residents it’s 9x or 6x and need 200k to get to BS so $22,222 or $26,667 usd according to my calculations.

I’ve noticed airfares seem to be higher for next year especially between full and business. Last year the gap was as low as 800, this year 1000, next year more than 1200 so roughly 3k round trip. So after 7 roundtrips in biz one would qualify for BS and get 14 upgrades for the rest of the year. These changes seem to benefit international flyers which they have better margins with per passenger.

BS 22,222/14=1,587.286 per upgrade.
Black 11,111/8=1,388.875 per upgrade.

So I ask myself if BS worth it. If they want more BS they should have a separate lounge or something like UA that has a separate lounge for international business.