Some fraction of people will reduce their workload so their earnings are $3000/month, bringing their total income back to $4000.
Some fraction of people will not reduce their workload at all and inflate their lifestyle to depend on $5000/month.
The overwhelming majority of people will choose the latter; we already know this, it's one of the chief complaints about a materialist, consumerist oriented society. When people get a raise, they overwhelmingly spend it. It's why most people can't save a $1000 emergency fund.
If you are earning $2000 per month and suddenly your income increases by 50% you feel like a baller, you are going to start eating better, ordering take out a little bit, replace that cracked phone, add a little more data to your cell phone plan, turn that thermostat up a little, whatever.