For example, if the written form of a language requires that a sentence end with ".", "?" or "!", and each one is an evidential particle, then tomorrow someone on the internet will say "By the way everyone, I'm tired of doing all of this evidentiality stuff when I don't need to, so I'm just going to write '_' at the end of all of my sentences, and it doesn't mean anything but that the sentence is over."

Actually ".", "?" and "!" are illoctutionary operators. Sentence have a different meaning if you use a different one in English. Yet we don't see anybody writing _ because he doesn't want to specify one of the three.

When it comes to the sentence "You are angry" I don't like the ugly copula. At the same time I still don't write "You angry" instead to have less effort because I'm lazy. After reading Science and Sanity I started to accidently drop the copula from time to time and write "there" instead of "there's" but the conventions of the English language still encourage me to not modify the language and write proper English.

In Chinese and Esperanto you can say "It rains" in one word. In English it would be easier to say "Rains" but that wouldn't be correct English and lazyness is not enough to get people to make that change. .

While I agree that mandatory evidentials are a tempting idea (I'd certainly like a language that has them), I don't believe that they'd hold up well in actual use.

I think you underrate the usefulness of evidentials because English doesn't have them.Via Saphir Worf it also will get easier to think in evidentials when you have a language that does them by default.

In the case of a conlang, part of the beauty comes from the fact of its widespread use.

I don't think you will get people to adopt a new language by focusing on the lowest common denominator. There's no reason to switch to another language that does roughly the same as English.

Esperanto get's part of it's charm from the fact that it's speakers treasure it. It's the most successful conlang despite the fact that it uses nonascii letters. While there are nonascii alternatives, Esperanto speaker still value using the original characters.