A move to plastic can work for a range but it's not always smooth sailing. From the examples you quoted.
Warmahordes didn't have stellar minis to begin with (IMHO) and some of their initial attempts at plastic were pretty dire. The game survived to be sure but from my experience Warmahordes players are all about the rules and system, the minis are very much a secondary concern behind gameplay.
Malifaux required an entire redesign of the range from the ground up. Wyrd have done some stellar work with their plastics to the point where I think they've surpassed GW. The only issue I have is the required complexity of the assembly to match what they want from detail leads to some madly fiddlily assembly, 11 parts for one human sized mini...
Steamforged are an interesting case and one worth using for comparison. They started as all metal but have just released 2 fully plastic (PVC) teams Farmers and Engineers. The latter two teams have a slightly different asthetic to the older teams which facilitates their easier production in plastic. The older teams are still available in metal and probability will be for the foreseeable future as Steamforged dont feel they can successfully recast them in PVC.
The very interesting part are two of the teams that appeared in the Kickoff Starter set. These teams (Brewers and Masons) were previously metal minis remade in coloured PVC. The comparison between the metal and PVC versions shows they are very similar but small changes were made to facilitate easier casting in PVC. In addition (and here's where it's relevant to Aristea) the kickoff minis are preassembled in China after casting from multiple parts, this means that the minis have allot of detail and dynamic poses for PVC minis since they don't have to be single piece casts.
Having seen the Guild Ball plastics, the bare metal Aristea sculpts and looking at the Aristea mini previews I highly suspect CB have used the same manufacturer as Steamforged and have gone through a similar process.
If they have I guarantee you that apart from some small flash issues the Aristea minis will be very high quality and you might be hard pressed to tell a primed PVC mini from a primed metal one.
Steamforged have announced that all the teams will be plastic by the end of 2018. I love metal minis and don't want CB to switch so believe me when I say that in the case of Guild Ball the switch to plastic will be a huge improvement. The metal they use is so soft that it bends at the slightest knock and the connections are often tiny and poorly designed. Even the older and more awkward Infinity models I've assembled have been much better designed and cast.
GB is also essentially a board game. The actual minis arent really the point, unlike Infinity where the aesthetic of the miniatures and table are a huge part of the fun, the fantastic ruleset is a nice bonus.
I'm sure I'm not the only one who buys Infinity models for factions they don't play just cos they're beautiful, in fact I'd bet a majority of players have one or two. I don't think many people do that for Guild Ball.