You don't really get around that with convergent applications either. That is, unless your idea of convergence is "lowest common denominator for everyone".

(it usually is)

More than doubles it, because there are a variety of phone sizes, tablets, monitors, &c. But that's OK! Build the best thing, don't settle.

The choice is usually not between building a great thing or a sub-par thing, but between building anything at all or not bothering.

... rather than doubling both the UI dev effort and the domain logic effort.