Hmm, yup I do agree you should take all of your assets in your custody, but having all the funds offline which is cold storage is not possible for everyone, as you've mentioned small or big doesn't matter but it does, I've seen people with the portfolio which is not even able to afford a cold storage device, I'm not aware of latest pricing still it's expensive. In my view if you own a small assets portfolio, it's better to switch your funds in self-custody hot wallets like Metamask, Trust Wallet, Electrum, and many other names.
Make sure to check the reputation before choosing as well.
All coins on a cold storage is definitely possible just that it comes at a cost of uneasiness, if you’re someone that actually do transaction’s everyday you can still have the funds on cold storage and just use a watch only wallet on hot storage to broadcast the transaction, although I get your point that it is less easy and more so the cost for them is another concern for someone storing a small amount. That’s why there is self custody hot wallets like you said but still for more easy people still go for exchanges which is the warning here. The risk is not the same with hot wallets.
I get that some people are day traders and need this exchanges most especially future or derivative exchanges, my advice to this set of people is have minimal funds on this exchanges or simply use more than one exchange because they can’t all be doing same thing at the same time.