Maybe you can educate me here but for me its been the other way around. You can always go down to 1/4 or 3/8, from 1/2" so why not start with the bigger. I try and by only 1/2" and then either go down with an adapter, or use a 10mm in 1/2". Plus all the air tools use 1/2".
I do have a cheapo 3/8" ratchet set with small sockets, in the event i need them but not invested in them.
While it's true you can go down from 1/2", it's overkill for most things. Most household and automotive can be done with 3/8" and 1/4". It's not true that most air tools are 1/2" unless you're thinking of the typical 1/2" impacts used to loosen lug nuts. Aside from lug nuts and bigger stuff like suspension (ball joints etc) I rarely used a 1/2" impact. My 1/4" air ratchet and 3/8" air ratchet and impact got lots of use. Especially once I got my 3/8" impact (IR timax).
A lot would depend on what you're doing. If you're working on semi's, by all means invest in 1/2", 3/4" etc. (keep in mind 3/4 and 1" impacts take a LOT of air volume. One of my coworkers had a 1" impact for really stubborn stuff and in under a minute or two completely drained our main compressor that otherwise supplied 3 bays including 2 air lifts). For around the house, unless you're doing major decking or something 3/8 and 1/4" drive are more practical.
The problem working on automotive with mostly 1/2" all the time, for one it can be too much in terms of torque (breaking things) for another it's too big to fit most places around an engine compartment and the like. This is often times where 1/4" drive wins out as well because it can fit places 3/8" can't. In trying to think of household uses for wrenches, I've used pipe wrenches and pliers for things like plumbing - combination wrenches for removing/installing toilets - screwdrivers - 3/8 sockets for putting together the random end table or kitchen table if the legs bolt on. Not sure how often people really need to do all that.
Most of this is assuming that people have air compressors, which many won't when first starting out. Even a small 3gal pancake compressor can run $100-200. While they're good for finish nailers and small staplers, you have to consider how they're used - an air gun like that uses a small rapid discharge of air intermittently. An air ratchet or worse yet, impact, relies on a steady volume of air a smaller compressor may not be able to keep up with. Then you have to consider the compressor needs to be near a plug and you're limited by the length of your air hose. Sometimes the air hose can be a pain, getting in the way which is why you see a lot of auto techs using cordless tools now.
All I know is, from years of working on cars I'd be beating my head against the wall if most of what I had to work with was 1/2". I invested in a wide range of 3/8 and 1/4 drive short and deep wells with adapters going both ways and those probably sufficed for 85% of the work I did 50hrs a week. Space constraints are typically the worst enemy, whether you're under a dash, under the hood or trying to snake a mish mash of extensions clear up to some obscure nut or bolt. (fyi, this is where locking extensions pay off - nothing ***** worse than several extensions/swivels and having them pop apart at the most inconvenient time). Kind of like trying to put together a picture frame with 16d nails and a 3lb sledge.
