The decay is lowered with power strength, and is reasonable. The decay is there so you don't keep insane damage easily.
The cap is there because they upped the rate per infected. If they increased it it would be a huge buff to what it was before. It's so you don't need so many enemies infected to get a decent rate up.
I do want it to stack with more enemies still, maybe with huge diminishing returns after 10 but something .
Death by spores back to a small radius I would like as well . (They did it this way though because the thing that kept super high damage numbers and instant killing is that damage went back to 0 when the spores got strong enough to kill everything almost instantly, but now it just decays and its possible to get up to that peak numbers right away instead of the strain dying off and they dont want the strain to spread when its strong enough to kill almost instantly)
Wait, if saryn isnt an energy hog, then why do you need energy on lash ?
The removal of cast on molt is almost the entire reason for the initial changes, adding it back would promote the stationary turret play, and it would be a little too easy to keep high damage strains of spore going, which is what they want to also avoid as well.
What you are suggesting is basically making saryn even easier to play, with no down sides and even higher damage and up time ... She doesn't need all of that.