Not really, the base game is well worth it on its own, especially at sale price
Plus, is there a lot in the DLC that hasn't been added back in the Steam Workshop?
not entirely sure I follow the question. But if I'm reading you right, yes, a lot of DLC features and "quality of life" improvements originally started as mods, which are still available.
Development needs resources. I don't see why there is so much hate towards well crafted DLC content. The base game is great as it is, and nobody forces you to buy the DLC.
DLC released every few months which adds new features or mechanics is perfectly fine. I'm not sure if its still the case by Skyline's team used to release DLC in two parts, a bit for a free and an extra bit you had to pay for, they do it really really well.
Generally there's a stigma around DLC cut out of the original game to be sold as extras on top, See anything EA / Ubisoft crap out.
It's the standard way for games developed or published by Paradox. It helps to diversify the gameplay after some time and you choose what you want to add from all the DLC.
The issue is that DLC is an umbrella term that applies to both exploitative micro-transactions as well as what used to be known as "expansions"
As someone who played the base game when it came out, it was a complete game at the time and doesn't require DLC to be fun.