The quickswitch does much more than cheesy axe kills. That's far from the point of the weapon, and people who rely on it are going to have a lot of trouble against scoots and people who can aim in the air.
Spies don't generally get more than 2 kills in a row anymore unless the other team is stupid, and he can get into a place where he can get kills that actually matter. A pyro is always going to be noticed and still takes longer to kill someone even with the lame axe combo, so again, comes down to how stupid the other team is. Pyro shouldn't just light people on fire and start swinging an axe because then it'll just get killed right away by any team who isn't garbage.
Picking off scouts and other players will
usually rely on your secondary weapon at the time or depending how close they are to you. It's not exactly advisable to chase a scout with an axstinguisher for obvious reasons.
I know the quickswitch can also help get to your secondary weapon faster as well. It's efficient in juggling and in need of a quick weapon change.
You kind of have to rely on your secondary weapon as well if a situation gets tricky so in a sense it all depends how your load out is really. It's not 100% relying on the axstinguisher/degreaser.
I know of spies that get more than 2 kills in a row, doesn't really mean the other team is stupid.
It just means they took the element of surprise to their advantage combined with other techniques like strafing or air stabbing from behind.
When you do try to light people on fire and melee, you have to use your best judgment.
I'm not saying someone should do that on a row of heavies because even I know that's suicide.
Also pyros aren't always noticeable when they flank and catch you off guard. There were many cases even I got combo'ed by a flanking pyro that seemed to come out of nowhere.
Just like spies use their judgment to go for a kill or not, it's basically the same with pyros.