I think it would be intuitive for people to see which network they are connected to in metamask if the ui changed colors or updated in some way to indicate the network like a logo next to the metamask logo. There shouldn’t be anything stopping someone from sending to a valid address but some extra visual cues on the tx screen would be very helpful.
I’ve already seen a couple users who have mistakenly sent a transaction to a CEX address while they were not on the ethereum mainnet which got me thinking about this.