That argument doesn't make sense because once the baby turns 2, you have to buy them a seat. So there goes the "affordability" question. Lap babies aren't an option after that point. So if anyone in the family gets sick or a parent has to travel in a hurry, they're going to have to buy their child a seat on the plane. End of story.
Don't judge people who wont make the same decision you would. If they wont fly with their baby in their lap, it's their right. I don't always buy organic food for my children but I congratulate those who do and still think it's better. I've also flown with my baby in my lap but only a limited number of times and only because either my car seat wasn't allowed on a foreign airline or once, as a FTM, I didn't make the reservation. My baby ended up without a car seat on a long drive, which was stupid of me and while my son is fine today, I don't want others to take the risk if they can avoid it.
It's valid to let parents, some who might be new to this game, know that flying with a lap baby is not safe. It's like the breast vs. formula debate. It's not being "judge-y" to say that something safe is better. Parent can assess the risks for themselves. Commercial air travel is very safe so the risks of something going wrong are very minimal, much less than formula or going anywhere by car or a million other things. But it's only fair that people know that yes, having your child on your lap a safety compromise.
If you are flying with a lap baby and need your car seat at your destination NEVER CHECK IT AS LUGGAGE. By all means BRING IT TO THE GATE. There are two advantages to this. First, if the flight isn't full, perhaps you can get a free seat next to you and bring your car seat on board. Best of both worlds! Your child is much safer and you didn't pay a penny!!
Gate-checking a car seat is no guarantee and definitely a compromise but there are fewer risks. By taking the seat to the gate, you're not letting the airline handle the car seat for as long, so less risk of it being sent to another city. At the gate, the gate-checked stuff is loaded directly on to the aircraft. They're usually the last items put on board so less chance of their being crushed by 10,000 suitcases. Wheelchairs are gate-checked.
Things can still go wrong. Some airports use chutes, which basically throw the gate-checked things down on to the tarmac. Also, someone could forget or run out of time and gate-checked items have been left behind on the jetway but that's rare.
What you're trying to reduce is the chance of having to leave the airport in a car without the car seat. The risk of something going wrong on the flight are far, far less than having your child unrestrained in a car.