Chewing is good for your teeth and jaws. I would argue that meat that is soft enough to melt in your mouth is not paleo, unless it's pure fat, or tongue, or raw liver.
Having said that, in my country grass-fed is the norm, and grain-finished is rare and expensive (that's what happens when you don't have massive government subsidies on corn). Maybe my standards are lower than yours, but I don't find I'm troubled by tough meat often if I pay attention in purchase and preparation.
Toughness is basically a function of these things:
- age of the animal (older = tougher)
- handling immediately before slaughter (stress = toughness and off-flavours)
- handling of the carcase immediately after slaughter (poor chilling practise = toughness, electrical stimulation of the carcase can reduce post-slaughter toughness)
- aging of the meat (the longer it's aged, the more tender it is)
- cut (muscles that do a lot of work and have a lot of connective tissue are tough, muscles that do little work are tender)
Of these things, you can control somewhat at purchase: choose a good supplier who has proper handling practises, choose a supplier who ages their beef, choose tender cuts. You can also do some aging in cool dry place or maybe your refrigerator yourself.
If meat is tough, then you can:
- pound it with a meat hammer
- grind it into mince
- slice it fine
- marinate it in an acid marinade, maybe for 24 hours. This isn't going to do much for thick pieces, to be honest, because it can't get inside a big cut very far.
- marinate it in a marinade with pineapple, papaya or kiwifruit. Watch out, these have protein-dissolving enzymes that can turn a steak into mush in 24 hours, don't leave meat too long with this kind of marinade.
- cook long and slow to dissolve collagen in the connective tissue into gelatin. This is not gonna work too well on a tough steak that doesn't have much connective tissue to begin with, it will work better on brisket or chuck.
- eat it rare or raw if it's a tender cut to start with
- combine one or more of these methods
Right now I have a couple of porterhouse steaks in the fridge, about 3/4 inch thick. They were probably aged 1-2 weeks by my local supermarket's supplier and they are dry-aging here for a few more days. By US standards they are hopelessly unmarbled, but I expect that done my way (sprinkled with flaked salt and pepper, seared 3-4 minutes on each side and rested for 5 mins) they will be quite tender at near rare.
Get Harold McGee's On Food and Cooking out of your public library and read the chapter on meat. It has a good user-friendly breakdown.