Decent? yes
imho, chess is one of those "activities" that you can be decent at without studying it (as in joining a club, hiring an instructor, buying "bibles")
you just need a lot of playing experience to pick up things on your own. Although you will definitely grow slower than a person studying, chances are, the few things you learn actually stay with you. Someone who learns just by reading will forget a few of what he learns, since theres just too many to take in at once
It's just like civ, regarding to the kinds of players that play a lot more than they study(read articles here, join discussions) vs players that like to discuss/read more than they actually play
The few things that could be detrimental to the growth and continuation of a potential player, are reading "bibles"
To some new people, it's just like reading the whole thickness of a yellow page just to ride a bike/ride a car
sometimes, too much information can be detrimental, when you can just take the first step yourself and learn from there
can I get reasonably proficient at chess just playing against a computer and not formally learning strategy or tactics or opening or anything?
one thing that greatly help anyone in anything, is by learning the concepts & principles by heart
By knowing the concepts & principles, you can make sound decisions no matter the situation
You will realize that openings, variations, mid game, and end are just a series of sound decisions + an open eye/mind
Although you will start having problems when you move on from Classical to hyper modern principles