It's not just an assumption, it actually is the default on iOS, Android, iPadOS, and macOS, and the user has to jump through hoops to get around it. I'm not saying this is a good thing, but having a name for it makes it easier to describe, search for, lobby for, and ultimately use. It's like rooting or jailbreaking or even something as innocent as a plug-in or forking: different techniques of modifying software functionality away from the default, but each with its own costs and benefits.
If you tell someone to install Adguard, for example, then on iOS they're installing an app that gives them a Safari extension through the official mechanisms. It can block ads in Safari. But on Android they don't install it from the official store, they sideload an APK by disabling some built in system protections against unsanctioned apps. It's more hoops on Android because of the sideloading, but they get way more power (it acts as a VPN and also a HTTPS MITM cert and filters all your traffic). It can filter ads in all apps, even through HTTPS. The downside is that now all your data goes to this small ad blocking company and circumvents the billions of dollars Google's spent trying to keep your device safe. It's a meaningful difference and users should know both the risks and benefits of being able to sideload apps, just like they should understand the risks and benefits of fully rooting your phone.
I see what you're trying to say, but the fact that it is already the default for many operating systems is all the more reason that you should stop tacitly accepting it, not the other way around
the term sideload gives implicit primacy to monopolistic app stores. install does not.
we should be fighting for third party installation to be the default, not to have the scraps of some workaround that developers are less likely to work with.
for 20 years computers managed just fine with third party downloads as the default. developers favoured broad compatibility and users had more power
there are times and places for locked-down devices: children. old people. corporate devices. display models. etc.
however, this should be opt-in. with an optional app store if the OS manufacturer cares to provide one.
using the term "install" favours a world where developers make apps that you can install on your own terms. using the term "sideload" favours a world where developers make apps for an official app store.
I'm not saying stop using the word sideload, I'm just saying stop fighting for sideloading, and fight for (mobile) install rights.
I don't entirely agree or disagree with what you're saying... depends on the target audience, I think? There doesn't have to be a one-size-fits-all balance between power & protection.
For myself, as a dev, I like Android for this very reason. I don't root it anymore, but I do sideload apps. But for friends and family, I have the exact opposite advice: Not only do I tell them not to sideload anything, they also shouldn't download any no-name apps from the official app store either (especially on Android) because the chance of malware and scams is so incredibly high.
This was always a risky thing to do, even back in the Tucows/CNET Downloads days, and why Windows malware was always a problem. Normal users don't really have the technical ability to audit all the random crap that's available on the internet. Hell, most devs don't either (how many npm or apt-get packages have I audited recently...? zero).
I'm the kind of person who turns off UAC for every fresh Windows install, and if they ever lock down macOS like iOS, I'm gone. But people like us also are not typical users. I think it's actually nice that the market offers this sort of choice. Power users and devs can always choose Framework machines and build their own Linuxes. Boring ol' web devs can use standard Macs. Kids and grandmas get iPads (and honestly most adults would be fine with them) in its default locked-down state. People who really want some customizability on their phones can still use Android and either sideload or put some de-Googled distro on there.
Really, the average person doesn't need this much power and customizability either for their PC, phone, TV, car, washing machine... they're just commodity appliances to watch Game of Thrones and browse TikTok on. I think it's actually a good thing they're locked down. They don't have to worry as much about all the malware and broken configurations that were so common in the Windows 95 days.
But it's also nice, of course, that there are still niche options in the market that you can get if you really want that full bare-metal experience. Most people just don't need or care about that, and it actively hurts them by making them less safe.
If you tell someone to install Adguard, for example, then on iOS they're installing an app that gives them a Safari extension through the official mechanisms. It can block ads in Safari. But on Android they don't install it from the official store, they sideload an APK by disabling some built in system protections against unsanctioned apps. It's more hoops on Android because of the sideloading, but they get way more power (it acts as a VPN and also a HTTPS MITM cert and filters all your traffic). It can filter ads in all apps, even through HTTPS. The downside is that now all your data goes to this small ad blocking company and circumvents the billions of dollars Google's spent trying to keep your device safe. It's a meaningful difference and users should know both the risks and benefits of being able to sideload apps, just like they should understand the risks and benefits of fully rooting your phone.