What to do with a 2 year old on an airplane who won't wear his seat belt during take-off/landing?

At the beginning of this year, I had the 'pleasure' of flying for the first time alone with my 2 year old son. It didn't go very well...it was a combination of waking up very early, tiredness, airplane being late, and karma. Karma because way before being a parent, I was flying behind this family with a toddler that would not stop crying...so I moved away from them first chance I got.
This time the roles were reversed, I was the one people were looking to move away from because my little one wouldn't stop crying. But that didn't bother me, different when you are a parent for some reason.

One thing that got me thinking was the wearing of seat belt. Because my son is over 2 years old, I have to buy him a seat and he has to sit in it. But he would not. It was my mistake because I strapped him in his seat belt as soon as we got on the plane but with it being delayed, by the time it was time for take-off he had had enough.
He threw a full-on tantrum when I was trying to get him in his seat and put the seat belt on because the flight attendants asked me to. I'm fairly strong but with all my strength I could not keep him in place with all the kicking and screaming. Same thing happened when it was time to land.
So what did I do? I held him in my arms both at take-off and landing, which clearly I'm not supposed to do nor do I want to for health & safety reasons. But I could not see any other way to it. The only reason I was able to hold him in my arms is because the flight attendants told me to put him in his seat belt but then they left and never came back so it's not like they were supervising me.

At the end of the summer, I had the REAL pleasure of flying alone with him once again. It all went smoothly because I learned from my mistakes. I left him free until the plane started moving which is when I put on his seat belt with no issues. Same when it was time to land.
But because I was anticipating and expecting some resistance, I asked the flight attendants when we were boarding the plane: "If my son will not stay in his seat wearing his seat belt, what can I do, do you have a seat belt I could attach to mine and hold him in my arms as you do with babies?"
They simply told me: "No, he is over 2 years old so he has to be in his own seat."

Thankfully it all went smoothly but I can't help but wonder if I had stricter flight attendants on the first flight, who demanded me to put on his seat belt despite me being physically unable to hold him down in his seat...what would've happened? It's nearly impossible to reason with a tantruming 2 year old. Would the plane not take-off/land until my son is wearing his seat belt? Would they kindly ask us to get off the plane (if during take-off)?

I'm just wondering if any other parents have had similar experiences? Or if you or someone you know is more familiar with the flying industry, what is the solution in such a situation, where you simply cannot get him to stay in his seat wearing his seat belt?