I think the thought process with Boss drops is that 1 scrap is > 0 scraps.
Well, the 50 to 150 is really a different issue than the increase in stamina cost. I think previous promotions actually had similar amounts of purples required, but let’s look at the expected cost for 10 stamina vs 8.
I haven’t bothered to record runs in PQ, but in DS the drop rate was somewhere in the 20-25% range for the right-most item (or newest). Assuming that is still the case here (and brief anecdotal experience corroborates this), then you’d expect to spend the following stamina for items:
50 scrap @ 8 stam: 218 raids, 1744 stamina
50 scraps @ 10 stam: 218 raids, 2180 stamina - an extra 436 (less than 4 stamina buys or roughly what one earns for free in a single day)
100 scraps @ 8 stam: 435 raids, 3480 stamina
100 scraps @ 10 stam: 435 raids, 4350 stamina - an extra 870 (roughly 7 stam buys or an extra 2 days of free stam)
150 scraps @ 8 stam: 653 raids, 5224 stamina
150 scraps @ 10 stam: 653 raids, 6530 stamina - an extra 1106 (roughly 9 stam buys or an extra 3 days of free stam)
If each hero has 1 purple item from the 10 stam area and you need to upgrade 15 heroes, that’s an extra 6,540 stamina required. If you upgrade all 32 heroes and each one takes just 1 item, that’s an extra 13,952 stamina. That’s 29 days worth of free stamina.