This mindset is why programs only ever seem to bloat more by the day. Code for a given functionality could cost 0 development maintenance and still not be worth including going forward.
I don't understand what you're saying.
Your presumption is that code is "cheap" because of LLMs. I am challenging the premise. Maintaining code in production for a product used by billions of people is not cheap, with or without LLMs. Perhaps your shitty disposable code that you write for yourself and never subject anyone else to is cheap, but if your code is in a large, complex codebase and has to actually work for everyone who uses it, not so much.
LLMs are only getting better and more competent, especially for code well-covered by tests. On such codebases it's definitely cheaper to maintain code with a LLM than by humans alone. I presume then that the XSLT implementation in Chrome is not well-tested.
I think that the issue is not the XSLT implementation (that could be contained), but its deep intégration in the XML stack which is part of web pages processing. Think about XML "processing instructions".