Personally, I’m not sure that in the long run Z working with Echo Plus will be that important, although most of the people in this forum will disagree with me. 
Over the past 10 years, Z wave has been wonderful for do it yourself people who were looking for home automation lighting systems that would be relatively inexpensive, fairly easy to install and wouldn’t suffer from interference with Wi-Fi. That’s really been Zwave’s strength. However, the networks are limited to a total of 232 devices (and as a practical matter, maybe closer to 150), and don’t have as good a battery life as zigbee devices.
In contrast, you could have thousands of Zigbee devices on one network (the main reason the lightbulb manufacturers have preferred it), you can have automatic discovery (which is what the echo plus is using for its smart setup), and really good battery life which is great for small sensors. But there’s that Wi-Fi interference issue, which can be a real problem for people doing their own installs who don’t have the kind of network mapping tools that the pros have.
Anyway, just the Wi-Fi issue has meant that Z wave has been much more popular in the do it yourself market, particularly for light switches. So most people in these forums, as well as in other forums for home automation which are aimed at budget do it yourself installs, have a lot of Z wave devices.
On the other hand, most people in the US in urban areas have some zigbee devices in their home already even though they don’t know it. Most cable TV settop boxes use zigbee, almost all utility smart meters use zigbee, and many sensor-based systems including medical monitoring systems and some irrigation systems use Zigbee. That doesn’t mean those will all work with SmartThings or Echo Plus, they won’t for other technical reasons, it just means there’s a lot of zigbee out there for things which are professionally installed. And the very expensive home automation brands like control 4 all use zigbee.
So if you go into a forum which is dominated by do-it-yourselfers who put low-cost as a high priority, you’re going to hear a lot about zwave. Everywhere else, not so much. 
At the present time, Z wave can’t do the “discover my devices” mechanism that echo uses. You have to at a minimum scan a QR code for each individual device. And that’s only for the very newest devices. With the older generations, adding a zwave device requires that each device be on power and that you do some physical manipulation with it, typically a button press pattern, in order to add it for the first time.
So my guess, which again is not a popular one in this particular forum, is that Amazon is never going to add Z wave to the echo plus. I may be wrong, but it just doesn’t match their “simple set up” requirement and I don’t think they need the protocol. And I don’t think they want the additional customer support headaches of dealing with two distinct protocols.
But I could be wrong. We’ll see.
What I will say that I think most people in this forum will agree with is that these days you should buy any home automation devices based on what they can do at the time that you buy them, not based on what they promise they might do some day, or what you suspect they might do some day. Otherwise you’re bound to be disappointed.