TL,DR: The official tools to keep up-to-date offline installers and extras are long overdue an update, removing the GOG Downloader option will make things even worst.

Being able to keep up-to-date offline installers of my library is a fundamental aspect of my purchases on gog.com.

Nowadays the bulk of my purchases here are titles I already bought and often played elsewhere, the idea of owning a working-without-depending-on-external-entities copy appealing enough to justify the double-dip.
The remaining purchases are games I'll eventually play months if not years down the line (when GOG may no longer be around, a sad but not impossible scenario) as I work through my backlog, picked up ahead of time to support the developers and GOG.

Keeping a sizeable library of offline installers and extras updated using the GOG Downloader is a tiresome process.

It doesn't help that in order to push Galaxy the download links were semi-hidden in a sub-menu, with no option to show them as default without 3rd party tools (and make sure to open the GOG Downloader links page on a different tab, or any title you may have already inspected but not necessarily yet added to the downloads will disappear from the list of games with an update available when you push the browser back button).
It doesn't help that the update notification system is a mess: updates notifications are often delayed and arrive in big groups, quite a few updates are not notified at all, some are notified before the links to the actually updated files are there, you can't choose to receive notifications only for specific languages or OS so you receive more notifications than you'd need, changelog entries are highly unreliable (many updates are not even reflected in the changelogs), so that overall the only reliable way to know what's been updated is through gogdb.org or the dedicated forum thread.
It doesn't help that the Extra Goodies section has become an "anything goes" repository where even full games installers for various operative systems or localised versions are thrown in, leading to significantly sized unneeded downloads and the need to manually check folders to avoid wasting storage space for stuff you've no use for.

Fact is, while keeping a sizeable library of offline installers and extras updated using the GOG Downloader is a tiresome process, doing the same with browser direct downloads or Galaxy is even more cumbersome and akin to a part-time job.

If you think a few additional mouse clicks to start each single download through the browser is no big deal, imagine what it means when the gog update notifications system wakes up from its slumber and throws at you 30-40 notifications in a couple of days.
Using Galaxy you may avoid some of the clicks, but then it will download everything no matter what (whereas the GOG Downloader just checks the downloaded file integrity if a file with the same name is already present in the download directory), and then you have to delete all the duplicate files.

I'm aware there are other, user-created options that would ease the process (gogrepo for instance), but I'm not comfortable using unofficial tools.

If you want to disable GOG Downloader, you should really update the remaining options to at very least truly match (ideally improve) the GOG Downloader capabilities, and take a serious look at the notifications system (and, to avoid misunderstandings, improve it, not just scrap it because "it doesn't match the level of quality we want to offer to our customers").