When my iPhone's leak, I'm out potentially $1500.
Of course, if you let your kid reassemble either your power train or your $1500 iPhone without supervision, the results will be exactly as you describe, but also entirely your own damn fault.
For that reason, I maintain precisely zero of them myself.
Apple devices (both laptops and phones) have a dramatically higher resale value than others, so that bit of your argument baffles me.
The reason why apple offers those services is because it has competition. If they figure out how to make third party repair intractable, you can expect a full reversion to the "screw you, buy a new device" state of affairs.
(Obviously telling the user that water proofing is compromised and that they need to retry attaching whatever they tried to attach or replace it.)