Distribution and publishing were indeed enormous costs, as was old school advertising and getting any traction.
All of those are costs and issues that are a tiny minuscule fraction of what they were then.
Costs, however, are irrelevant to value. Apple, via the App Store created a high value product by offering access to a lucrative market for less hassle and costs previously associated with bringing and selling a software product to market.
Developrs have never had it this good, IMHO. Apple's model lest them work on product development while Apple handles all the administrative and legal requirements related to selling the product, all for 15% for most developers. If Apple were to allow sideloading, I doubt most developers would find a better deal and access to the same market; they'd likely pay as much or more than Apple's fee and have a smaller market to boot. I'd also bet EPIC et. al. would not want to abandon the App Store for their own store. It's simply too lucrative.
Apple, in some ways, is a victim of its own success.
For ake of argument, let's suppose Apple decides to open up the iPhone to sideloading. At that point, IMHO, Apple becomes free to charge however they want for the App Store and setup rules governing allowing users to decide how apps can access their data and the OS. For example:
App Store
- Charging a monthly access fee to be on the store plus a per d/l fee
- Adopting an EPIC licensing model for access based on annual revenue from an app on the store
- Basing developer fee based on revenue from an app
- Fees to sign apps
- Offer the current 30/15% deal and terms as an alternative to the above
iOS
- Adopting a sandbox mode where users can block non-signed apps, much like the Mac
- Require apps to ask permission to access data or track, and Apple follows the same rules. No tracking, no popup, track, popup required
- Not be required to offer choices initially for any alternative to Apple apps
- Not allow unsigned apps to access parts of the OS/device designed to maintain security
Those are just some random thoughts. I'm not saying Apple would do any of that, nor am I against sideloading, per se, my point is I doubt sideloading / alternate app stores will be the the nirvana some think it will be; and small developers may find it harder to bring apps to market and make money.
You could see an expansion of the SetApp iOS model to more apps. The challenge for developers is how much money do they make from being on SetApp. The monthly fee is hardly enough to pay more than a few pennies per install, they seem to be the Spotify of MacOS apps.
One unanswered question is "How will sideloading impact piracy?" The App Store hasn't eliminated it but it is much less easier than in the jailbreaking days. Even then, jailbreaking was more of a hard core user thing, sideloading could open up the jailbreaking world to the casual user. If it becomes a problem to the point of seriously impacting developer revenue, I suspect we'll see more subscription based apps rather than one time purchases as developers try to protect their revenue. From a user perspective, more subscription based apps is a bad thing. Even worse would be more ad supported apps.
How will this all play out, who knows, but I think this is a be careful what you wish for because your wish may just get granted situation.