Early 2015 MacBook 12 won't run Monterey

I think the best way to answer your concern is to do a quick review of Apple's support categories.

They are:

• Supported - Products are consider supported when they have been distributed for sale within the last 5 years.
• Vintage - Products are considered vintage when Apple stopped distributing them for sale more than 5 and less than 7 years ago.
• Obsolete - Products are considered obsolete when Apple stopped distributing them for sale more than 7 years ago.

So, "doing the math," your 2015 is now in the Vintage category. Since it can still run macOS Big Sur, there is no big hurry to replace it. FWIW, I still have 2010 (Obsolete; runs macOS High Sierra) & 2011 (Obsolete; also on High Sierra) in my possession. Both currently work quite well. I did; however, retire my 2008 MacBook (OS X Lion) as it could not keep up with today's security requirements.

As you know, Apple no longer sells the MacBook model. Whether or not, they decide to do so in the future, who knows. I just bought a pair of M1 MacBook Airs for my family and they love them ... so, whether or not, you want to get a new notebook, well, that's totally up to you.

In the case of my 2008 MacBook one easy indicator was that I could no longer visit secured (https) websites as TLS 1.2 support did not become available until OS X Mavericks ... which couldn't run on this notebook.

Basically, Apple does a good job providing security updates for most of their macOS versions. However, as you can imagine, the older OS X versions will eventually not be supported. For now, I would not recommend any before High Sierra and what I use, currently, as my threshold. If the Mac can't run, at least, this version, it's time to part ways with it. Of course, if you don't have a need to go beyond your local network, you can certainly run much older Macs. I do.