intuition

While I am usually a Debbie-downer on these topics, I am not sure about this speculation.

First, they need new ticket types as they are introducing Y+. So announcing new types is not primarily driven by a wish to cut benefits.
Secondly, they also need new types as the current scheme is falling apart. It was all invented before the big cartels came about, and we all know that the cartel leaders didn't care about AY ticket types, so the system immediately got pruned to only work on the few non-cartel destinations.

With that said, Y+ has to fit in somewhere in the earnings tables. But if Y+ pushes Y earnings down, then it is unlikely to also push J earnings down. It has to wedge in between them. If they build a system where Y earns up to 100% and J earns from 100% and up, then there will be a huge overlap when Y+ wedges in.

However, I do agree that Finnair is driven by their wish for ancillary revenue, and cares less about traditional revenue. Ie managers are likely praised for reaching a ancillary target and forgiven for not reaching the traditional target.
"Hey, ancillary is up! Good work, here is your bonus!"
"Hey, total revenue is down, but that is likely due to industry trend so no need to blame AY manager decisions"

Combined with what has been said to investors, I am pretty sure the new structure is focused on cutting benefits from tickets and selling them back, at a premium. And that this will apply to all class of service. So we are very likely to see a "J minus", which is not cheaper in terms of what you pay, it is just a really cheap product...