Vertical integration doesn't decrease competition, it increases it. If there are N companies in a market, and a company that purchases from one decides to vertically integrate.and enter the market, now there are N+1.
Almost no one who vertically integrates competes in the market they just shook. You can't get Amazon to ship things for you, so they don't compete with Fedex and UPS. You can't by an M1 chip from Apple.
Vertical integration is all about shrinking markets, not adding to them.
What can Apple make? What must Apple buy from other suppliers? How much are investors allowed to punish Apple's stock price (like they are currently encountering https://www.forbes.com/sites/qai/2023/01/08/why-is-apple-sto... ) or can they buy everything a company produces? Are others allowed to buy a company that only has Apple as a client and raise the prices?
Can Amazon work at improving distribution center efficiency with technology that they create? Or must they buy it from someone else?
Must Google create a business unit to sell its highly customized routers?
Almost no one who vertically integrates competes in the market they just shook. You can't get Amazon to ship things for you, so they don't compete with Fedex and UPS. You can't by an M1 chip from Apple.
Vertical integration is all about shrinking markets, not adding to them.