I think it's a calculated move on Subaru's part.
To do what, you ask? Well, I think they're holding out HP for future model updates. Why would they go ahead and make it 300 HP right out of the gate on a new model, when they can do incremental power increases, over a 5-6 year model life, and use that to generate more sales?
We know that this engine's displacement can easily support quite a bit more horsepower and torque. And, COBB and other Subaru performance aftermarket folks will happily step up and offer that. And, the fact that this engine is under-tuned from the factory will mean you'll be able to get big gains with very limited bolt-ons. Reliably.
The last WRX was barely changed over a 6-year model life. Why? Because it sold very, very well. There was no need for them to do anything to it.
Enthusiasts online are a vocal group, and are not at all shy with criticism. But there aren't very many people talking about the fact that this car is, really, in a class by itself. Even its 2 closest competitors, price-wise, the Civic Si (205 HP for 2020 model) (actually not even made for 2021, but the 2020 model was $25,200 MSRP), and the VW GTI (228 HP) ($28,695), are front-wheel drive only, and don't hold a candle to even the last gen WRX, performance-wise.
It is the only sports sedan available anywhere near its price point that is not front-wheel drive, and which offers a manual transmission. The 2021 model starting MSRP is $27,495, which handily undercuts the GTI, perhaps its closest natural near-peer, and easily out-guns it, performance-wise. That makes it a tremendous value.
So, please - tell me again why Subaru would dramatically increase engine power?
You want a more powerful sports sedan? You can get one. Just pony up the $37,245 for a 2021 STi, or $44,640 for a 2022 Golf R!