Sites already built on top of multiple subdomains: Unless there’s an important reason to maintain a multi-subdomain strategy, for sites that want to provide a unified experience it’s worth considering migration to a single domain. If that’s not an option, other workarounds mentioned before should be explored to achieve a consistent UX.
This is a collection of advice which makes a lot of assumptions. It’s just been brought up at work as a reason to build a monolith.
I’m infuriated that things like CORS are put up as barriers when it’s very little maintenance or overhead.
The ServiceWorkers themselves will be much more work than these few line changes, but you’ve ignored their limitations and focused on easily solvable points.
You have a point RE: sub-domains for locale, I prefer a path-based, query string based or cookie based locale to sub-domain routing, but that has nothing to do with complexity, it’s just basic common sense that sub-domains should be for domain separated entities like payroll vs calendar, website vs stock-management.
All of this is to AVOID making a monolith which is hard to maintain and difficult to reason about quickly, and your advice is guiding people right back towards monoliths.