I can't full agree with you on this, because if a person has a monthly income of $2000 then investing 50% may not be that difficult for them, if they are conscious of their spending and stop all waste, they can easily invest 50%.
I think everyone is right from their point of view here. If we are realistic we can see that among earners there are different types of spenders. For an individual, who is unmarried and has no one else fully dependent, it is possible to invest 50% or more and most are easy. He has people to help him in times of trouble.
I don't think all of them can be right in that contest because allocating @Ricardo11 made a comment that is vague and somehow contradictory to what you said. He did not give conditions that will make someone who earns $2000 to invest 50% in Bitcoin like you try to give rather he is looking at the amount and not considering the responsibilities attached to the money. There are countries where monthly rent is as high as $500 which when removed from the $2000 will mean the investor will be left with $500 to cover for every other thing including food, transportation, medicine etc and is still expected to set aside emergency funds. How will it be possible to invest $50% under this condition?
I feel that the percentage will always vary per individual based on what we are comfortable with and what will give us the needed peace of mind that will not put us under any pressures to sell