To clarify: not everyone who has back pain has a rubbery spine, but everyone with a rubbery spine eventually gets back pain. Usually, when strength decreases. For example many women gymnasts are fine during their career up till 25yo or so, then by 35yo they're a mess - they haven't done the sport for a decade, have popped out a baby or two (weakening abs etc), etc.
If you get stronger than "untrained" then you will be whippier. "Whip" is simply power. Power is force exerted over a distance... quickly.
Your quickness is largely genetic, but obviously knowing where the ball is coming from and being there first is a practiced skill.
What we're talking about here is strength - the ability to exert force. So if you don't practice your "whip" then yes you'll slow down.
But if you get stronger you'll speed up.
This is why you do not give up your sport entirely while building your strength base, but simply reduce the intensity of practice so it doesn't eat up your recovery resources which could be going to getting stronger.
Whip = power = force x distance / time
Distance is not a variable we can control, it's a function of your limb length, how the ball's coming at you, etc. Time is as I said partly genetic - you're just fast or slow - and partly practiced skill. This leaves force, which is very trainable.
This is why I would duck when Serena Williams lobs one at me.