I don't have any aphids on my tomato or pepper plants; all grown in earth boxes or root pouches. I use the Seabright yellow sticky cards on every plant. (Brand is very important as others are death traps to good things like lizards)
I think a healthy mature plant should be able to resist aphids.
Since early 2017 season (drought) I do get the Tomato Russet Mites (only seen larvae and moving larvae this year, none of the ugly adults. I am struggling with that with my latest test adding neem meal to the mix and top dressing the mature plants in root pouches with the neem meal. No adverse side effects.
You might want to get a little scope to see if there is anything you are missing.
I use this one; it is good/cheap:
https://www.amazon.com/Carson-MicroBrite-60x-120x-Microscope-MM-300/dp/B00LAX52IQ/ref=sr_1_2?
ie=UTF8&qid=1520555740&sr=8-2&keywords=magnifying+scope
When I'm at the end of the lifespan of cucumber plants, I get tons of aphid eggs. I pull the plant b/c it isn't worth the hassle treating it.