Those who subscribe to the service don't stop buying games, quite the contrary. Gamepass users seem to buy more games than those who don't use the service (
According to Spencer at Giant Bomb interview).
I'm sorry but I think this is a case where people have somewhat misunderstood what Spencer is saying and have run a mile with it. Forgetting what he has said for a moment, it is very difficult to believe that sales will stick or even rise as subscriptions rise. This has been established clearly by other services in similar areas, both with sales of music as streaming services like spotify have come about, and with sales of dvs/blurays as streaming services like Netflix have come about. It would be
extraordinary for subscriptions to lead to more sales in the long run based on this, and based on fairly simple logic: subscribers are unlikely to buy games they already have access to. In the longer term, the effect ramps up again, because a user stops buying products that
aren't available through subscription because they're satisfied by what
is.
Game Pass subscribers buying more games does not imply that subscriptions lead to purchasing more games (basic correlation vs causation issue). There are several reasons why that could be true that seem more logical:
1) Game Pass subscribers tend to be people that play more games anyway
2) Game Pass subscribers end up gaming more on Xbox rather than other platforms
3) A short term Game Pass subscriber buys a game because their subscription lapses (a common case in the aftermath of one of these big discounts I'd think)
None of the above reasons would lead to a situation where sales could be sustained if Game Pass (and similar subscription services) become widespread in the way they have in music, film and TV. As it happens, in the case of Gears, Benji has indicated that Game Pass seems to have cannibalised day one sales, something which hadn't been indicated with previous launches on Game Pass. I would expect that to be common. Game Pass is going to have upsides and downsides like these in the long run, but I expect for Microsoft any cannibalisation in sales will be more than made up for by the additional subscribers.
As I keep repeating, I'm positive about Game Pass and its future. But the notion that it's going to be all up and subscription services won't affect game sales is a fantasy.