I do this all the time on Alaska. I have the same Peak Design travel backpack as a sibling comment mentioned, plus an ordinary overhead roller suitcase. Maybe one in five times I have to gate-check the suitcase, but the rest of the time I have ~90L of space carried on and off the plane.
How is it inconsiderate? The suitcase goes in the overhead where I'm taking up no more space than any other passenger, the backpack fits under the seat in front of me. It squashes my legs a bit, but the only person getting inconvenienced there is myself.
1. You've literally described an overhead bag that exceeds the maximum allowable carry-on dimensions at pretty much every airline, so you're taking more than your share of overhead space.
2. Flights get slowed down for everyone when people are exceeding the underseat bag size limits - sometimes the bags do get checked in the metal sizers at the gate, which causes obvious slowdowns, and 300 passengers with 45L underseat bags are going to take significantly longer to board/deplane than 300 passengers with <24L underseat bags.
3. I'm skeptical that you can actually fit a 45L bag under plane seats without ever inconveniencing other passengers. Seats are pretty tight and it's pretty easy for a larger bag to push your legs sideways so they cross the midpoint of the armrest. And various planes don't have dividers between every under-seat area, so if your bag is oversized it's easy for your bag and/or feet to spill into your neighbour's under-seat area. But maybe you're small or very disciplined and are able keep your body and belongings in your own seat area, I've (probably) never sat next to you, so can't say for sure.
e.g. If you max out the allowed dimensions with United you get a 45.08L carry on and a 23.65L personal item.
And most "cheapest tickets" now no longer include a carry on bag, only a personal item.