Ending support for Windows 10 could send 240 million computers to the landfill. Why not install Linux on them?

It depends on the definition of "support ended". Like, there are various forms of extended support that you can pay for for versions of Windows, and some companies do.

https://en.wikipedia.org/wiki/Windows\_XP#Support\_lifecycle

Support for the original release of Windows XP (without a service pack) ended on August 30, 2005.[4] Both Windows XP Service Pack 1 and 1a were retired on October 10, 2006,[4] and both Windows 2000 and Windows XP SP2 reached their end of support on July 13, 2010, about 24 months after the launch of Windows XP Service Pack 3.[4] The company stopped general licensing of Windows XP to OEMs and terminated retail sales of the operating system on June 30, 2008, 17 months after the release of Windows Vista.[114] However, an exception was announced on April 3, 2008, for OEMs producing what it defined as "ultra low-cost personal computers", particularly netbooks, until one year after the availability of Windows 7 on October 22, 2009. Analysts felt that the move was primarily intended to compete against Linux-based netbooks, although Microsoft's Kevin Hutz stated that the decision was due to apparent market demand for low-end computers with Windows.[115]

So for those, we're all definitely a decade past the end of normal support. However, they have their extended support packages that can be purchased, and we aren't a decade past the end of those...but most users probably aren't actually getting those:

On April 14, 2009, Windows XP exited mainstream support and entered the extended support phase; Microsoft continued to provide security updates every month for Windows XP, however, free technical support, warranty claims, and design changes were no longer being offered. Extended support ended on April 8, 2014, over 12 years after the release of Windows XP; normally Microsoft products have a support life cycle of only 10 years.[118] Beyond the final security updates released on April 8, no more security patches or support information are provided for XP free-of-charge; "critical patches" will still be created, and made available only to customers subscribing to a paid "Custom Support" plan.[119] As it is a Windows component, all versions of Internet Explorer for Windows XP also became unsupported.[120]

In January 2014, it was estimated that more than 95% of the 3 million automated teller machines in the world were still running Windows XP (which largely replaced IBM's OS/2 as the predominant operating system on ATMs); ATMs have an average lifecycle of between seven and ten years, but some have had lifecycles as long as 15. Plans were being made by several ATM vendors and their customers to migrate to Windows 7-based systems over the course of 2014, while vendors have also considered the possibility of using Linux-based platforms in the future to give them more flexibility for support lifecycles, and the ATM Industry Association (ATMIA) has since endorsed Windows 10 as a further replacement.[121] However, ATMs typically run the embedded variant of Windows XP, which was supported through January 2016.[122] As of May 2017, around 60% of the 220,000 ATMs in India still run Windows XP.[123]

Furthermore, at least 49% of all computers in China still ran XP at the beginning of 2014. These holdouts were influenced by several factors; prices of genuine copies of later versions of Windows in the country are high, while Ni Guangnan of the Chinese Academy of Sciences warned that Windows 8 could allegedly expose users to surveillance by the United States government,[124] and the Chinese government banned the purchase of Windows 8 products for government use in May 2014 in protest of Microsoft's inability to provide "guaranteed" support.[125] The government also had concerns that the impending end of support could affect their anti-piracy initiatives with Microsoft, as users would simply pirate newer versions rather than purchasing them legally. As such, government officials formally requested that Microsoft extend the support period for XP for these reasons. While Microsoft did not comply with their requests, a number of major Chinese software developers, such as Lenovo, Kingsoft and Tencent, will provide free support and resources for Chinese users migrating from XP.[126] Several governments, in particular those of the Netherlands and the United Kingdom, elected to negotiate "Custom Support" plans with Microsoft for their continued, internal use of Windows XP; the British government's deal lasted for a year, and also covered support for Office 2003 (which reached end-of-life the same day) and cost £5.5 million.[127]

For the typical, individual end user, one probably wants to have been off Windows XP by 2008.

You're ignoring the fact that they ended XP support for months and then brought it back for literal years after so much outcry