Is my furnace really dead or is this guy upselling me?

A cracked heat exchanger diagnosis made without pulling the burners or getting a camera in there is not a real diagnosis. Any tech worth paying will kill the power, pull the burner assembly, and look at the exchanger cells with a flashlight, or run a camera through them. Some use a smoke pencil or combustion test to check for leaks. That takes twenty minutes, not a ten minute glance at the outside of the cabinet. And refusing to show you the evidence is the biggest red flag in your whole story. There is nothing secret about looking at a cracked exchanger. Good techs show homeowners that stuff all the time because it closes the sale honestly.

Not sure how much this helps but our furnace started doing the on-off-on thing last winter and it turned out to be a dirty flame sensor, like a ten minute fix. So the short cycling part alone doesn't automatically mean the worst, there's cheaper stuff that causes it too.

When I did end up getting a second opinon on somthing else, I just flat out told the company up front I'd had a bad experiance and wanted to see whatever they saw. The tech was totally fine with it, took pictures on his phone even. Might be worth saying that when yours shows up Thursday so they know you're not going to be difficult, just that you want proof.

First time posting here by the way, so sorry if I'm missing some rule.

I'd push back on one bit from that first reply though. Saying a tech who shows you the evidence is closing the sale honestly isn't always true. My old neighbor got shown a 'crack' once, big visual presentation and everythign, and it turned out to be a rust line along a seam that two other companies said was fine. Some of these sales shops know exactly what to point at to scare you. The commission structure matters more than wether they let you look. I'd care less about the dog and pony show and more about whether the second company sends a tech who isn't paid to sell you a unit that day. Ask Thursday's crew if they do diagnostics-only calls or if everyone there works on commission.

That on-off-on pattern is classic overheating, ussually the high limit tripping because air isn't moving through the furnace like it should. Clogged filter, weak blower motor, too many closed registers, all of it can cause the exact symptom you're describing. Mine did the same dance at 17 years old and the culprit was a filter I'd forgotten about during a kitchen reno, cost me nothing but embarrassment. When the Thursday tech comes, ask them to check airflow and static pressure before anyone says the word replacement. Even if the unit is genuinely near the end of its life at 19 years, that call should come after a real diagnosis, and ask for an itemized quote because labor is a huge chunk of any install price.

Update: second opinion came out and it was a dirty flame sensor plus a filter that was way overdue. They cleaned it, swapped the filter, and the short cycling stopped. No crack, nothing. They actually pulled the burners and showed me like the first reply said a real tech should. Can't beleive the first company tried to sell me a whole furnace.