They CAN choose NOT to use Chrome. They used Windows' browser before and no browser component before that. Steam doesn't NEED an embedded browser, having one just makes things more convenient for Valve because they can reuse their website in the client.

They CAN maintain support for older versions in their fork of Chrome. Browsers are expansive but not rocket science. Valve has the funds to maintain one.

They CAN make a specced down version of Steam (or GUI for steamcmd) for older operating systems. They don't have to keep selling Games on older operating systems (or could require an external browser for that) but taking away the ability to run already sold ones is unacceptable.

Problems caused by middleware are still the responsibility of the developer using that middleware. The end user does not and should not have to care how you cobble together your software.

They have simply decided that doing the right thing would be more costly than whatever cost they will incur from dropping support. That only works out for them because people accept this shit rather than demanding Valve keep their promise that you can continue to play the games you bought from them without trying to add additional conditions.