Good points on the ethics of web performance according to those that care about accessibility and energy use. Most don't enough to redo/switch apps. I find that people trying to achieve political change usually fail since they target their messages to people that already agree with them or a tiny number on the fence. I don't even bring this stuff up any more unless I'm talking to people that already profess to care about it who may have overlooked an extra opportunity to improve things.
Let's redo this in a way that targets most peddlers of web apps. We tell them there's still lots of users (give big number) who have hardware or network connections that make their apps unusable. Show examples like with charts and/or pictures of half-rendered content. Remind them they're trying to maximize growth, revenue, etc. That means they should design their apps in a way that effortlessly pulls as many people in as possible. Tell them specific practices make their site perform well for about everyone. Show examples illustrating the difference. Then, encourage them to do what's in their best interest to help them get famous, rich, etc. That's how you do it!
If they're on a budget, you can also show them how certain tech on backend are several times faster which lets them use cheaper VM's. They save money immediately followed by scaled savings during future growth. If the VM cost is OK, I've also found that to be a way to sneak in something else: they trade one VM for 2-4 in different, geographical areas that improve speed of content delivery and service availability. To be honest, I also warn them clustered setup is more complex to manage.