I am done with that after all the crap that previous software with admin rights installed. I cleaned it up and swore I would not install anything that required admin rights again - for my own security and peace of mind. What does it need admin rights for?
Why do I need to give Adobe admin rights to install Photoshop? Why do I need to give Microsoft the same to install Word? And now for a web browser?
If I'm installing something low-level like a window manager or a keyboard shortcut tool, I get it. But for a normal mainstream flagship consumer application, what on earth do they need admin privileges for?!
I hate the fact I have to hand over the keys to my computer just to run basic industry standard software and I have no choice because it's industry standard and I have requirements to use it.
On the other hand, for most people this admin vs regular user separation is almost meaningless and provides little extra security. Sure, if I install malware as admin it will be harder to get rid of it, but except for that, it can hurt me just as much, since all of the files I really care about (documents, photos, game saves etc) are already accessible with my user, and any program running as my user can already connect to the internet and send information about what my user is doing (not to mention bother me with ads). Some of the really damaging ransomware that recently made the rounds didn't even require admin privileges, it simply encrypted data in some common user-owned folders, if I'm not mistaken (it probably did need some privilege escalation to spread, though, which is a big problem on office networks).
So all they should require is to be dragged into the /Applications folder (or /System/Applications in Catalina). That's it.
That's how a macOS app should be installed. That should be the end of the process unless it needs to install kernel extensions (and even then, from Catalina onwards, such extensions should become userland extensions).
Applications also don't need to be (and third party apps on most single-user systems arguably shouldn't be) in /Applications. Users can put apps wherever the heck they like (I keep mine in ~/Applications which even gets the same icon automatically if you create it).
Apple could really streamline this because all apps that can be drag-drop installed really belong in one's own home directory, where they'll be seamlessly migrated when you move to a new computer, or brought with you if you have a roaming profile. As it is right now, I just keep both Applications folders in my Finder Sidebar and remember that the bottom one is "mine."
In an ideal scenario, users could still drag apps to the "one" Applications folder and if they are not an Admin the app itself would actually be stored in a default-invisible ~/Applications. Then when you opened "/Applications" the system would show you the superset of both locations.
This is of course nearly exactly how the Windows Start Menu works except that it always had "installers" do the work of putting those program shortcuts in the respective directories, and only well-behaved apps ever asked which place you wanted apps installed.
The question of how a regular user could be allowed to install something for all users in this fashion, placing the app bundle in the /Applications folder, is no different than when using an installer: ask for an administrator password.
On Windows at least, UWP apps are deduplicated across all users on the same system so when user #2 installs the same app already installed by user #1, it will just point to the same existing shared instance of the app's files. I don't know much about MacOS but I'd guess there's a similar packaging system. These systems also incorporate sandboxes which address the other problems you mentioned in your last paragraph.
The problem is just when the OS teams can't convince other developers (even ones inside the same company in Microsoft's case) to actually use their packaging and sandbox systems.
It wouldn't be wasteful if I didn't want to install it for the other users, which I might not.
In this case, it's installing Edge and the universal Microsoft AutoUpdater app. Given the range of stuff that thing has to support (including all the Office apps) and the privs it will need to run at to do so, I think the admin requirement is not entirely unreasonable.