You're meeting your users where they're at. If half your visitors are using mobile and half are on desktop, then having two designs isn't 2x the work, it IS the work. Having only one means you're abandoning half your visitors and not doing your job fully.
And having both is not really 1.5x to 2x the work, in my experience. Maybe more like 25% to 30%. A lot of components and widgets can be reused, the typography and colors can be similar, and certain screens can keep their layout with just small tweaks.
Sure, maybe the initial design takes a bit longer since each screen needs several breakpoints. But that's only a small part of the overall design work anyway. Then on an ongoing basis, your previously established patterns (and components in code) can largely be reused with small tweaks, easily done with modern toolkits like Tailwind or MUI.
I don't think it's that big a deal. Web devs have been doing mobile designs for more than a decade now, and the tools have gotten better and better. Honestly, it's way less wasteful than Agile ceremonies or endless meetings. If you want to stay lean and cut cruft, take it from places that don't directly affect the user, not the one place where they actually use your product all day.