no, you are assuming your use case is universally valid.
Sure you can stream a 4k movie using 4g but if you wanted to download another one simultaneously and/or have someone else in the house using the network to stream, listen to music, game, or download anything, you will run out of headroom quickly. With multiple users on the network 40 Mbps gets congested very quickly.
Then of course there is the issue of latency. if you game the network will be considerably slower, not to mention the eons it would take to download massive updates or new games.
Then there is the issue of congestion on mobile networks, especially in populous areas where many users don't generally have access to a terrestrial connection,
I run a smart home with over 50 connected devices, latency can affect that. I also upload large files regularly for work, a 4g sim generally has much slower upload than download speeds. Your cheap solution works for you, but it doesn't work for everybody. not by a long shot