Yes, unless you upgrade your host and for some reason it cripples your plugin. Now that Sonar and Waves have worked out the VST3 niggles between themselves it would seem to be fine for the time being.
Additionally, as long as you have "version 9" (the current one) you will always be able to install the latest sub version even when your WUP runs out. It's just the new full version that you'd need to pay for. Additionally, if you purchase WUP for your product, you get a voucher that counts towards any other plugin or bundle purchase. Also, upgrading any plug or bundle to a larger bundle automatically renews WUP for all plugins.
You don't NEED to buy WUP anyway, unless you upgrade other parts of your system and your older version won't work with it. Also, Waves adds plugins to bundles from time to time. You get those for free while WUP lasts, and if you don't have WUP but then purchase/renew it you will get all plugins that have since been added to the bundle.
It's also on sale sometimes.
I don't get all the vitriol. It's not cheap, true, but neither is Sonar or any of the other software we buy/use. In a professional environment it's a write off and not even a particularly big one.