Should I buy a new phone?

So, I've had my iPhone 6s for almost two years now.
My storage is full and the screen is super cracked (recent happening). Apps are still of course running and fine but sometimes I do see that I've had it for two years now.
I won't update my software cause of it being laggy.
I've heard people say that people should buy iPhones every 2 years.
Should I consider getting a new phone?

I would.

As far as software iOS 12 is fine for the 6s (may breathe new life into it).

No

If your phone were still in good condition I'd say you can probably get another year or two out of it provided you clear up the space. I finally had to get rid of my iPhone 6 after about 4-ish years when it started having display issues. It had no physical damage beyond a few dings to the edges.

The thing is of course that your phone sounds like it's not in good condition so it's not going to last that long probably.

The two year thing is mostly because phone contracts last two years traditionally so after that most people are eligible for an upgrade.

You probably are also eligible for one through your carrier at this point. The screen cracks are going to get worse to the point where it may stop working entirely, and at that point you'll have no choice but get a new phone or pay an exorbitant fee to repair it.

People should buy things when THEY, decide it is time to buy them. So whether your phone is two or six years old, prefect condition or like yours is now, doesn't matter. When the current phone stops serving YOUR needs, it's time to either get a new phone or get something else.

So does it being laggy tip the cart for you, or the fact there are no longer updates to the OS? You can always make room on the phone by moving all pictures, videos and music to your computer or an online storage option or external hard drive. That way you still have access to the old stuff, and plenty of space on the actual phone, for new media.

First check if you have AppleCare or insurance.

Even people who sell iPhones are unlikely to say they shouldn't last 4 years. At least since cellphone service providers stopped putting the price of the phone into the bill whether your purchased a new one or not.

If you do replace it, get a phone with at least 32gb internal storage.
The only reason my wife replaced her iPhone 6 is it had a cracked screen, so now I have an iPod Touch with a cracked screen.

Since it is broken, If you don't have an Accident Plan nor Insurance then you might want to see what promotions Apple or your service provider or your next service provider have.
If it wasn't broken you probably could get a year or more out of it until Apple stops supporting the software