Hi Jeroen,
I don't know whether you realize how confrontational your email sounds, and how difficult it is to start a friendly and constructive conversation with such beginning. Since this is an open, welcoming, and friendly mailing list, I ask you please to consider this important factor in future communications.
The MediaWiki codebase powers one of the top Internet websites and thousands more, with a wide variety of use cases. It also supports APIs, extensions, gadgets, templates and more, allowing for the development of very different features and usages. Nobody denies that the MediaWiki codebase needs attention and focused work, in fact this is why this team is being created, but there must be something good in it that made it arrive to this point.
It is difficult to agree or disagree with your blunt and very vague critique. Still, at the end you ask a good question: what will be different this time?
Reading https://www.mediawiki.org/wiki/MediaWiki_Platform_team there is one sentence that shows what in my humble opinion is a key difference:
Our approach recognizes that MediaWiki is a product in its own right and
deserves a dedicated
roadmap, product manager, and engineering team with a sense of product
ownership.
The initial team has four developers that know the MediaWiki codebase very well, and a dedicated product manager is being hired https://boards.greenhouse.io/wikimedia/jobs/613548?gh_src=1u385n1#.WOI0pogxAt0. It has been a while since these developers or anyone could work full time maintaining MediaWiki core as a whole. The page also mentions a focus on paying off technical debt, on working on platform dependencies of Community Wishlist wishes, on treating feature development teams as customers... and I don't recall any of these being an explicit goal in the past.
I think all this is great news. I am looking forward to see the new team in action, and the first results of their work.