You are reading incorrectly; you can have skills running with a total combined rating of 18, that is, 3 rating 6 'softs (or more at a lower rating of course).

doesn'T matter anyway, as changing the skill is a free action, as long as you have it loaded available on internal storage space somewhere in your body linked up to the wires.
at least that's what it was in SR3 and probably SR4 too . . might need active wifi to do it in SR5. because. you know. cables don't work without a wifi signal to the matrix at all.

Yes, sorry, my mistake. It's only a total of 12, not 18. 18 is with the wireless bonus included.

Unless they went went back to having to worry about offline storage space, then all you have to do is get the skillsofts.

you can have a skilljack rating 6 at chargen. it lets you know stuff. which is not significantly more useful than the ability to do matrix searches about stuff, or owning a datasoft that has information about stuff.

all for the low, low price of 120,000 nuyen (or a 'mere' 90,000 nuyen if you get it used).

you can't have skillwires at a rating higher than 4 (and that only if you get it used, otherwise you max out at 3). and that will cost you 60,000 credits (for whichever one you choose, used rating 4 or new rating 3).

and in general, that isn't going to substitute for a main skill. it might let you sub in for some niche skills, but then again, there are often better ways to get those niche skills (like hiring someone to use those skills on your behalf when you need them, or just having those skills in the first place).

after all, do you really want to be unable to spend edge on a demolitions test? how about a test to successfully land a semiorbital flight?

The problem is the cost, as we've gone over a million times someone thought that skillwires and jacks should be priced out of existence for 99% of the Shadowrun universe. They are essentially unobtainable at a usable level unless you commit a ridiculous portion of your starting nuyen to them, and in-game I'm not sure how normal NPCs are ever supposed to have them unless their corp supplies the wires at cost.

Like I said, Knowledge skills have different levels of importance at different tables. Getting a level 6 skilljack and never having to worry about not speaking the language in an area you are sent to would be nice for merc-type characters, but probably not super important for most Seattle-based games. It's all just another one of those head-scratchers that SR5 added for no particular reason anyone can discern.