He's preaching the same thing many before him have. It's a real problem. Some of the older aircraft are money pits due to their repair and maintenance costs. Then there was the RIF a few years back that took out thousands of experienced maintainers.
The most important stat of all to Commanders...Fully Mission Capable or FMC. Meaning ready to fight. Most are happy to get 75% FMC rates for their squadrons...the other 25% or more are down due to maintenance.
In 2018, General Mattis "ordered" services to achieve an 80% or better FMC rate. They couldn't do it. The USAF fleet wide was at about 72% FMC.
https://www.airforcetimes.com/news/your-air-force/2018/03/05/fewer-planes-are-ready-to-fly-air-force-mission-capable-rates-decline-amid-pilot-crisis/Many manufacturer's of the original parts are long out of business and so cannibalization from retired aircraft until there's none left to be had only gets you so far. The B-52 J-57 engines are only still running because there are so many variants of the engines used around the world, that "core" parts aren't too difficult to get.
The KC10, is on average, 36 years old. The KC-135 is even older entering service in 1957 and both are to be replaced with the KC-46. However, Boeing has been doing a wonderful job of delivering aircraft that are behind schedule and with shoddy maintenance. Fortunately, most of the overruns are on them and not the taxpayer.
It's not a simple "taxpayers get screwed" scenario...with a limited amount of $$ to spread across the services, service chiefs have to be creative in how they fund things. Are you aware that the services have to pay for retiree medical care? Think of what could be done with that $$ to support the active components if the funding was coming from another bucket of money. (And before some of you retirees on this forum lose your $hit, I am USAF retired.) In 2013, of DoD’s $150 billion request for compensation, healthcare costs were ~$40B of that...that's a lot of money that could be spent where it's needed, on the active force.