If you look at the WP34s (and WP31s) project, that literally took years to be "complete". Sure, there were early versions of the software in people's hand much sooner, but that was a DIY repurposing project, not a final product sold to the public as a business venture. As such, "early rough edges" and some significant ongoing development could be tolerated more easily by the customer.
You can see SM worked on the DM42 for quite a while before they beta'd it and then more time before it was officially sold. With a proposed 43s project, leveraging much of the developed DM42 hardware would hopefully means that portion could happen quicker, but now the software development will (I would venture) be more work since there's no finely polished Core baseline like Thomas Okken's Free42 to start from.
It's an ambitious project, but we've seen how nice a job both a) the WP34s design / UI / software team did, and b) the DM42 hardware team did, so I really look forward to the fruit of their efforts.
As creative and skilled the team(s) may be, the 43s will be a whole lot of work for a small team working part-time.
My guess is that it will take a while (i.e. multi-year), so if you need a calculator any time soon or fancy the DM42, go ahead and get one now. Presumably the final 43s will be so similar in user form (especially that crucial Numpad / Enter↑ / Math operator area! ) that no one will be deterred from getting one in addition to any existing DM42(s). One for home and one for work!