The problem is that group (1) results in a nightmarish race-to-the-bottom. File creators have
zero incentive to create spec-compliant files, because there's no penalty for creating corrupted files. In practice this means a large proportion of documents are going to end up corrupt. Does it open in Chrome? Great, ship it! The file format is no longer the specification, but it has now become a wild guess at whatever weird garbage the incumbent is still willing to accept. This makes it virtually impossible to write a new parser, because the file format suddenly
has no specification.
On the other hand, imagine a world where Chrome would slowly start to phase out its quirks modes. Something like a yellow address bar and a "Chrome cannot guarantee the safety of your data on this website, as the website is malformed" warning message. Turn it into a red bar and a "click to continue" after 10 years, remove it altogether after 20 years. Suddenly it's no longer that one weird customer who is complaining, but everyone - including your manager. Your mistakes are painfully obvious during development, so you have a pretty good incentive to properly follow the spec. You make a mistake on a prominent page and the CTO sees it? Well, guess you'll be adding an XHTML validator to your CI pipeline next week!
It is very tempting to write a lenient parser when you are just one small fish in a big ecosystem, but over time it will inevitably lead to the degradation of that very ecosystem. You need some kind of standards body to publish a validating reference parser. And like it or not, Chrome is big enough that it can act as one for HTML.