Well, I don't know about you, but sometimes I'm on a 3G network. Yes, I could wait to get home. But some people also have really terribly bad providers offering them very little bandwidth.
So, by doing this, you make sure your content can reach more people. Which, to me, seems to be the whole point of making and putting it online in the first place
Of course, but there are instances of the opposite. I encounter both from time to time. For example I want to show someone a video while having a drink, usually it doesn't have to be crisp 4K 60fps, just play the god damn thing before we lose interest. Other times I want to watch a video, I'm in the middle of nowhere anyway, I have time and data in the monthly cap, just not enough bandwidth.
User choice and smart defaults would go a long way.
Well, people do expect YT/NF level. And sadly we've trained everyone to expect that everything starts playing instantly.
So if your income depends on people not rage-quitting your page, you'll have to provide something that satisfies their "needs", or more appropriately "desires".
I'm not saying people shouldn't do more for user experience, but the cost-benefit seems to be bimodal. Do the minimum, or pay YT/NF/Vimeo (or CF or AWS Streaming, or get ready to bankroll your own).
It is by no means a clean or modern solution (I agree, go with Youtube etc or provide a transcoding js player), but about a couple decades ago many sites would simply link to several resolutions of a video, allowing the user to play either a low, medium or high res version as per their current data connection. So that is one option that sidesteps adaptive bitrate players as well as a large CDN.
As has been pointed out above, having some kinds of video downrated to be streamable on 2G isn't much different from not having the content at all. While 4K is rarely required, pushing out coding tutorials at any resolution below 1080p is nearly worthless, and pushing it out at 480p is worthless (Unless it's a retro C64 BASIC tutorial in which case rock on).
Work... Yes, you can watch a video fluently, but it takes (on Desktop, Firefox, with Throttling on "Good 2G") maybe ~2min to load the video page. However, you can start watching the video before the full page is even loaded.
Obviously it's not going to be YouTube/Netflix-level anyhow.