I just hope they stick to Gecko even after upgrading Qt to 5.6.

Yes, there will be QtWebEngine (based on Blink of Chromium 45) in Qt 5.6, but once/if it is released then it won't be that difficult to rebase the alternative browsers (Webcat, WebPirate) on top of it. Yet it is quite unlikely there will be any community browsers based on Gecko and some variability is needed. Today, when something doesn't work in Webcat, it is almost certain it won't work in WebPirate either, yet if often works in Sailfish Browser (like IPython which is usable only with the default browser) and then there are cases where it's the other way around - therefore it would be sad to have all the browsers with the same engine.

There are also more up to date alternatives to the aging Gecko based browser on Fremantle, but I always prefered the robustness of it and flexibility of about:config. I hope SailfishOS keeps this legacy.