A word is enough for the wise and you have taken time to state it out, it is now left for those participating in such to stop. How can a wallet seeks us to store our assets without finding it worthwhile to embed backup phrase and we will use? This shows something is not right and caution should be applied. This is blockchain where decentralization reigns supreme, if there is no backup phrase then no need to use such wallet.
I guess that people using online wallets without backup phrase are novice or probably they just lack the knowledge of it, because no one will in his clear eyes store coin in a wallet without a backup phrase in this critical industry where we have lots of scammers, I wonder why some of these wallets too are implementing this type of technology without backup phrase plan if they are really serious and does not also have the plans of defrauding their users in future.
There are so many reliable wallets in the market that has backup phrase and can be used without stress or fear, I think what I would just do is to make a list of the ones I can guarantee and probably post it for them to see if we don’t have someone that has done it yet.