There are viable alternatives to Nexus including ModDB and others.
Just as viable as Gab and Dissenter?
ModDB is >#8000, Nexus around #1000, the gulf is huge.
There are games that have a Moddb site but no Nexus site, like the Total War series. It just so happens that the Nexus hosts mods for the most popular games out there, which naturally leads to more traffic.
And if you google "[insert game] mods" you will either get Nexus or Moddb as the first google result depending on what game it is.
So if you want to make mods for the most popular games out there, you can't avoid giving Nexus its cut.
The mob called, they're jealous of Nexus' business model.
Except that you can, it just so happens that the major mod communities congregated around certain websites for whatever reason. Convenience most likely.
The Total War mod community is centered on the totalwarcenter forums, which is a classical forum like RPG Codex. They don't have an integrated upload functionality, so modders there used to just upload wherever. Many old links are now dead because the filehosters are defunct, and a recent preservation effort caused people to reupload old mods to ModDB because that's a major mod hosting site with a pretty reliable track record.
The Neverwinter Nights mod community used to be hosted on the now defunct Neverwinter Vault hosted by IGN. When IGN took down its hosted mod sites, fans created a site rip and migrated everything to their own website. So now the only place where NWN mods are hosted centrally is
https://neverwintervault.org/
The Thief modding community, in which I am somewhat active in (got a few released fan missions under my belt), is centered around the TTLG forums and several central hosting sites have popped up and died out over the 20 years of Thief modding. My go-to download site for fan missions is
http://www.taffersparadise.co.uk/ but there are plenty of others, too. There are AT LEAST three different sites that host user-made objects and textures map designers can use in their own maps: Targa's House of Thief Stuff, Christine's Screenshots, and the Thief Object Repository. There has been a move to upload everything to the Thief Object Repository in recent years, but those other two sites containing objects made by Targa and Christine respectively, still exist too. Both authors are totally fine with their stuff being mirrored on the Object Repository btw. And central fan mission download sites like taffersparadise collect every fan mission ever made, they're not uploaded by the authors themselves but by the site's host. Mission authors upload their mission on a free filehoster like MEGA, post the link in their release thread on TTLG, and a few days later it will pop up on taffersparadise, archived for eternity. That's how it works and nobody has ever complained about it.
Of course, any modder can upload his stuff elsewhere if he wants.
For example, there are several Total War mods created by foreign communities that don't have official threads on TWCenter. Plenty of Russian mods, some Chinese, a few German. Finding them is a bit harder, especially the Russian ones that don't have download links outside of Russian forums (hope you can read Cyrillic bro). But the modders don't mind: they create these mods for themselves and for the small community of their forums, and if people are really interested they will find the mods. I was REALLY into TW mods for a while and made accounts on Russian and Italian forums just to download obscure foreign mods for the games.
You can totally make a Thief fan mission and not announce its release on TTLG. That just makes it harder for people to find it (but eventually, it will be found and uploaded to taffersparadise). Same with making a Neverwinter Nights mod and not uploading it to the NWN Vault.
The Nexus isn't nearly as central as you think it is. It's the central hub for Bethesda games, which are among the most popular mod-friendly games in existence, and that's why it's so big. The Nexus started as Morrowind Nexus and Oblivion Nexus and only expanded to other games later. Bethesda games made it what it is, and the current discussion about the Nexus' new rules is also dominated by Skyrim and Fallout 4 modders. It just so happens that the Bethesda modding community picked the Nexus as their central modding archive to upload their stuff to, and things evolved naturally from there.
But once you start looking into the communities of other heavily modded games, you will quickly find that there exists a massive modding world outside the Nexus.