But I hope you can imagine why a chat team does care if a technology doesn't allow them to send timely and reliable notifications.
It is often better to have no feature than a bad feature. I've never used the mobile web messenger, but if it is unreliable, only works in a few browsers, it makes sense to remove the product.
Sure, that's why they have the app. But it's not a reason to completely turn off messaging from the website. Heck, they could even turn off chrome notifications completely and the website would still be completely functional.
1) you can never assume a recipient ever has their phone on, with them and in a non-silent mode 2) users can disable push notifications for individual apps anyway
That said, the reasons you mention are pretty edge cases. Instead of just hating on it, just stop and think about it for a moment without prejudice. When you email someone you don't expect them to read it immediately, but when you send a text you at least expect them to know they received something, no? (If you say no to this, I really have nothing else to say anymore, you would be just denying the reality) That's what Facebook wants to be. They're competing with SMS, not email.
Also, if they're supporting desktop clients I again call shenanigans on your argument.
Shenanigans!
I am an occasional user of Facebook Messenger, and using the mobile website was perfect: I don't care if notifications are delayed or even lost (I'll see the messages eventually anyway). But there's no way I'm going to install a separate app for something I use maybe once a week, tops.
If it stops working from the mobile website, then I just wait until I'm back at my computer to read fb messenges.