Why do you feel you need to convince them of anything? I totally get having your eyes opened to the broader spectrum of coffees available but end of the day it's a preference based on subjective senses. Let 'em do them, what's the harm?
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Welcome to boomer psychology 101: you scream at them very hard and they will scream back and keep their position, but after you have left they will change their mind
Welcome to asshole logic 101- you must scream at people for absolutely inane reasons, such as their coffee being "too bitter".
Helping with maintenance tasks like cleaning the grinder is a good idea, but I wouldn't count on changing your parents' preferences. If you don't enjoy their preferred roast/ratio, just bring some of your own beans and make a separate pot for yourself when you visit. I had a similar situation with my in-laws and that is our solution. I bring my coffee when I visit them and they bring theirs when they visit us and we're all happy.
how do you convince someone that bitter coffee is not good coffee?
That's the neat part; you don't.
Without judging, some people confuse bitter for strong. You should try brewing their coffee different ways that will make it intense, but not bitter to see if they like it better. I'm not an expert, but maybe Chemex? Aeropress? Pour-over?
That's all you can do if they end up preferring ~~butter~~ bitter coffee!
Let people enjoy things, man.
Isn't taste also affected by age? Like eyesight and hearing, it diminishes with age.
Their coffee might taste to them the way your coffee tastes to you.
Simple, just respect their taste. I had friends who grew up with instant coffee and they hated even the best brewed coffet I could make..
I think you may be SOL.
If they like coffee this way, that may just be that. And it's okay. By all means try making them a proper cup of a nice dark roast, but at the end of the day, if they've come to expect bitterness outta their morning cup, you ain't gonna satisfy the habit with something that ain't terribly bitter.