Most frontends already display available crossposts so you're not wasting anything more than grabbing all the comment sections as well.

We're talking about different things. I'm talking about the view you get when you first open an app - the 'home' screen that lists the posts. The API response for api/v3/post/list doesn't indicate whether something has been crossposted. You can see for yourself by getting a list of the 2 oldest posts on lemmy.ml:

curl --request GET --url 'https://lemmy.ml/api/v3/post/list?type_=Local&sort=Old&page=1&limit=2' --header 'accept: application/json' | jq .

For those 2 posts, you can only find out if they have crossposts by individually querying each post using the api/v3/post endpoint - the first one in that list would be:

curl --request GET --url 'https://lemmy.ml/api/v3/post?id=2' --header 'accept: application/json' | jq .

where crossposts would be in the 'cross_posts' array.

So for an app to display whether a posts listed on the main feed have crossposts, they'd have to query post/list, and then for each entry, query /post as well. This isn't the way these things typically work - there's normally a 1-to-1 relationship between an API query, and displaying the results of that query on the page. Looping through the list you've been given, and making extra queries adds complexity and delay, when the expectation from the user is that this list should appear pretty quickly.

What you're talking about, is the view once a user has clicked on a post, not the post list. This provides the crossposts info. It's important to realise though, that the cross_posts array provides everything an app could want to display info about the other posts. It's not like they are pulling the data for one post, and then pulling data for each listed crosspost, so if they were to start getting the comments for each crosspost, that would be an extra effort (and a potential waste).

I don't agree at all. There's space for complex frontends which attempt to adjust the feed according to their own logic, as well as minimalistic frontends which follow the backend's design explicitly.

My counter to that, would be that if you aren't using the API in the way the developers expected, your app has ceased to be frontend, and is instead its own program that's scraping data from it. There are already some heavy desktop-orientated frontends, and none of them do what you're proposing. I think that the reason why, is because the proper way to do it is for the Lemmy's backend to be changed to provide the information they need in one go. That's unlikely to happen, but that doesn't mean that hacking away at an improper solution is necessarily the right answer (you just end up supporting a project that isn't supporting you in return).