In theory could we skip the proposed Taproot soft fork, activate Simplicity instead and use Simplicity to write the equivalent of Taproot scripts?

I think that's both very premature, and misses the point.

Simplicity is a completely different approach to scripting, with significant advantages, but it's being designed from the ground up. There is lots of progress, but it will require more than just the base layer.

You can't just write scripts in Simplicity, and if you have one, you can't just sign for it. There is work on improving that, by building something Miniscript-like for Simplicity, but it's all a long way from being usable & integratable.

And even ignoring all that, if Simplicity was completely ready and usable and reviewed and everything... it would be strongly preferable to integrate it as a leaf version under Taproot. Doing it otherwise would remove some of the incentives for hiding policies that Taproot offers.

This question was answered by Pieter Wuille on Twitter.