It takes virtually nothing to scale down. Again see WW2. Millions of the best workers in Europe, Asia, and America were dead. No one needed to put in overtime to not grow wheat for the dead.

If you reduce the population as fast as its decreasing now, lower than replacement rates, all modern conveniences including infrastructure

I don't want to keep saying it but WW2. The working population was decreased far faster than today. You don't need extra factory workers to not produce iPhones.

That includes the internet and hospitals and all internet-dependant companies.

I ran a large internet company. You don't need many people to serve millions of customers. If there were less people, there would be less equipment installed. Less customers means less employees needed. So service would not suffer. If anything it would improve. Because Internet servers/bandwidth is built with oversubscription built in. You don't really have 100mbs service. It's that peak usage for your local neighborhood is modeled such that as long as everyone isn't using maximum bandwidth all the time, you have the illusion of 100mbs at any time. Reducing customers over time means that existing networks and servers could handle unusual loads without slowdowns.

It's the same with hospitals. You don't need more doctors when you have less patients.

business because of the inability to get parts,

You don't need parts because you have less people that want to buy your product.

Throw in experienced power plant operators, people that install and maintain pipes and lines, water treatment plants, public transit

When German and Japanese factories were bombed and the experienced operators were killed, someone else was trained and took their place the next day.

As older people die, you get younger people to take their place. You don't need population growth for that. If anything, population decline means the younger generation gets better training because there are more of experienced people giving the fewer younger people their knowledge. Instead of one teacher with 50 students it can be 1 teacher with 10 students.

Population decline doesn't mean tomorrow 99.9% of the entire planet suddenly disappears tomorrow. We have had far more rapid population declines in history and the results have always been overwhelming positive.