I think you just missed some steam sales, but they will come around again.
Out of all the games you could buy though Skyrim is imho probably the best bang for buck you can get in its potential longevity. The modding community for The Elder Scrolls series is very seasoned in their collective experience, advanced at creating tools, and mod content adding more to the world you engage in has been refined over many years. Personally I have been around the forums since early Morrowind ( about ten years ), and its still not old .. my eldest re-installed it a few days ago on her laptop.
The above is the main reason you really ought to invest for, all mods need to be created with the Creation Kit ( CK )
The CK updates along with each official update patch to the game, so the two for want of a better word become meshed.
And mods created with the CK then become dependant upon users having the correct patch to the game .. Otherwise the game breaks in subtle and very complicated ways.
And of course the whole update of the game patches and CK are intimately tied in to steams drm / encryption.
No matter what any cracking group will tell you, they will never 100% be able to guarantee a fully compatible up to date hack of the game which will work with all the mods out there. Slight changes to the game code can mess with 'records' for example, which are being used by the mod creators. If there are ID changes your whole setup is out of sync, TES5Edit which we use for cleaning official content 'mistakes', relies on record format structures which the more advanced coders of our community pooled together to reverse engineer to make the tool work.
And some of our more advanced mod creators are conducting huge efforts such as the Unofficial Skyrim Patch in their own time ( have a look at the list of fixes
the USKP does.
USKP is the number one essential mod ( as is the UDGP for Dawnguard, and UHFP for Hearthfire ) .. But as you may have guessed by now, will only work with the latest game patch they are created for.
Glad you were 'hypothetically' honest anyway
, people trying to make mods work with non-official games usually try to hide the fact, and have the objective of finding a way to make a mod work .. Which wastes an awful lot of mod creators time scratching their heads over what the problem could be when they could be getting on with developing / solving problems that they need to, and not concentrating on one users 'problem' which will never be resolved without the official game setup.