So true! I use an old 2009 27 iMac as 3rd Display with Target Display Mode. On 2009/2010 models you can also connect an PC with Display Port (2011 and later only Thunderbolt). Still great display and good for environment not throwing away working tech stuff...This.
My old 2013 21' iMac had its internal hard drive fail (1TB spinner) but I was able to repurpose it as my Plex server by adding an external SSD. (the thing runs an order of magnitude faster than when it was new) And I bought a 2nd iMac in 2017 to replace it as my desktop system... but I use that older iMac as a 2nd monitor via "target display mode".
I love how that older iMac is serving two functions simultaneously.
View attachment 1983687