I don't believe that we should propose a "New core" team. The Platform proposal seems way too broad in scope to me, and @Ori would like to add Availability to the scope of the Wikimedia Performance Team. I may outline some more specific comments about the Platform proposal at Talk:Wikimedia Engineering/2016-17 Goals/New MediaWiki-focused team/Platform proposal, and I believe that we probably take further discussion on this topic over there.
Topic on Talk:Wikimedia Engineering/2016-17 Goals/New MediaWiki-focused team
The Platform proposal seems way too broad in scope to me
Opinions are obviously going to vary. I think the scope is relatively close to the scope which was once in favor when the (short lived) Wikimedia MediaWiki API Team was formed. The wording has changed to be less biased towards the Action API and more towards MediaWiki, but at least for me the ideas are fundamentally the same. I'd be glad to be proven wrong, but to date I don't see major refactoring and impactful shared feature work making a lot of progress under the verticals belonging to the Wikimedia Product group. These efforts are instead done on the backs of individuals who volunteer to design, defend, and implement such changes. It's great when it happens, but not a very stable system on top of which to build and advance the goals of one of the largest properties on the Internet.
The planned vision of creating platform focused teams within the Product verticals died quickly when the reality of budgets and growth caps were introduced. It's time to at least talk about how a organization with ~100 full time employees in engineering allocates resources to the foundational component upon which all of its features are built. Parsoid is a bridge tool to allow Visual Editor to manipulate wikitext. RESTBase is first and foremost a durable cache layer for Parsoid output. The native mobile apps are API clients of MediaWiki. Having a group concerned with strengthening the core product that manages all of the data and most of the business logic to facilitate collaborative document creation on the Wikimedia projects seems completely reasonable.
Yes, let's discuss that proposal there. In general though, I don't think problems with existing proposals are a good reason to not propose a new team. The proposals try to solve the problem outlined in https://etherpad.wikimedia.org/p/newcoreteam-unconf (MediaWiki is the centerpiece of the WMF's strategy, and compared to that the organizational support it gets is unreasonably low). If you think that's not a problem, or not a problem that can be improved by proposing a new team, or the problem will be solved by the Performance team partially redefining itself, those would be good reasons (but not very plausible ones IMO). If the problem is real and the existing proposals are not adequate, that should be solved by improving or reshaping the proposals.