After Bitcoin Cash split from the main Bitcoin blockchain, a plethora of other cryptocurrencies followed suit. First came Bitcoin Cash, then came Bitcoin Diamond, Bitcoin Gold, Bitcoin SV, and so on. Each fork is basically the same Bitcoin with an identical supply (21 million coins) and genesis block. What makes them different is their approach to scalability. Every Bitcoin fork that is created, essentially multiplies Bitcoin's total supply two or three times its size. This is sort of a double spend (although not in a direct manner). Imagine if we had 10 Bitcoin spinoffs, that means the supply will increase to 210,000! I'd bet if the Bitcoin Cash hard fork didn't happen, we'd still have only one Bitcoin (BTC) with a price higher than today. I'm still wrapping my head around this so please correct me if I'm wrong.
Do you think Bitcoin-based forks (spinoffs) will devalue the original Bitcoin (BTC) over the long term? If not, why? Don't we have too many forks already? Your input will be greatly appreciated. Thanks.

I don't think it will devalue Bitcoin. Maybe very slightly as those advocating the fork may not be ok with what's happening around BTC. But they also know that not having BTC in your portfolio is quite naive. Even Roger Ver I guarantee still holds a lot of BTC compared to his Bitcoin Cash holdings.
The only thing you can fork is the database. Everything that has been built around a certain coin, the whole ecosystem, community and partnerships and whatever you want to think of, simply can't be forked.