This is something that a lot of us do because it's fun, not because it's something that we have to do. We might make a module so that we can do something like this because it's fun, satisfying, and efficient (plus it looks good) …
echo $pages->find('brand=/brands/audi/, mpg>25')->renderCars();
But the reality is that it's also not necessary. You could perhaps more easily just package it into your include file and do this:
$cars = $pages->find('brand=/brands/audi/, mpg>25');
include("./render-cars.inc');
And in your render-cars.inc:
<?php
foreach($cars as $car) {
echo "<p>{$car->title}: {$car->mpg} mpg</p>";
}
What I'm trying to get across is that much of the things we all create and use modules for is optimization and fun. Perhaps others feel differently, but in my case, most of my sites don't even use any modules other than what comes with PW. I create modules when I want something I can re-use on multiple sites or share more easily with the PW community.
Another point about modules is that they are a whole lot simpler than you would ever imagine, once you get going with it. But the lack of ability to create modules is not going to limit your ability to use ProcessWire in any way. You can make it do pretty much anything without having to even know about modules. But when the time comes that you become interested in it, it will only increase your enjoyment of development. So when you see what appears to be complex development conversations about modules and such, don't worry. What you don't know isn't going to hold you back in ProcessWire. It already seems like you have a really good understanding of development and using ProcessWire. Based on your past posts, I feel confident you can push ProcessWire to do what you need when you want to. And we're always here to help with questions and problem solving.
If I were you, I would keep using ProcessWire for everything that you are comfortable using it for. When situations come up that you feel can't be as easily solved with ProcessWire, then investigate services like what Pete mentioned (Lemonstand, Shopify, Google, Facebook, Flickr). There are services out there for nearly everything and this is where a lot of functionality is trending. For instance, look at the quality of a comments service like Disqus... it makes you wonder if we aren't far off from the time when built-in comments are no longer considered a required core feature of CMSs. When we had to setup a forum for ProcessWire, I never considered trying to create it myself in PW. Instead, we went with SMF, and then IP.Board. Services like these and others are better than what you can reasonably expect to build on your own, or what you could expect to come with (or be added-on to) any other CMS.
Honestly, if you use ProcessWire and then utilize services for the things you don't want to build, then you will be able to do everything you could ever want. And more quickly, more securely and easier to support, than if you were trying to leave it all to a CMS. For the rare cases where you need something that won't be easy to do in PW, and your service options are limited, then bring WordPress into the mix. Not that WordPress can do much on it's own, but it's following is so much bigger than anything else that literally every possible thing has been coded for it. I certainly wouldn't want to use WordPress as my CMS, but I have no qualms about pulling it in when something that I need is available as a WordPress plugin. You aren't going to find any other CMS that has as much 3rd party stuff built for it. WordPress is easy-enough to figure out in a day (from a development perspective) that you also won't find yourself as frustrated as in Drupal (at least, this was my experience). It's not much prettier than Drupal from an output generation perspective, but it will be much more respectful of your time.