I'm guessing we'll see that too. When, I don't know... But probably sooner rather than later. Generally, Garmin's (and everyone else's) gadgets last between 6-12 years on the market. 430 from 1999-2005, 430W 2005-2011 (you could still get it after that, but the GTN replaced it). The G1000 came out in 2004, the NXi at the very beginning of 2017. G600 came out in 2008 and G500 in 2009, and their TXi versions in late 2017. GTN 650/750 in 2011, Xi models in early 2020.
At least part of this is driven by component availability. If you wanted to build a GNS 430 today... You couldn't. No matter how much money you had, unless you wanted to open up an old-school chip fab in the process. The components that made up the 430 are simply no longer available. As technology in general goes, so goes avionics.
The G3X Touch came out for the experimental market in early 2014, so we just crossed the 6-year threshold about a month ago. I don't expect it'll be replaced this year since it's still relatively new to the certified market. The G5 came to certified in summer 2016, the next thing resembling it was the GI 275 earlier this year. So, figure a 3-4 year lag, and I expect we'll see a replacement for the G3X Touch - Or at least, a newer, nicer gadget that fits in the same market segment - Sometime around 2022-2023, and absolutely no later than the end of 2026.
Not sure what you mean by that, but the G3X is still a first-generation product. I expect we'll see something with a capacitive (as opposed to infrared) touchscreen with a "retina" resolution and faster processor. I also hope the bezel will shrink a LOT - That's probably my least favorite thing about the current G3X Touch is the amount of plastic around the edges that could have been used as screen space.
There's always an upgrade in the works... But if you always wait for it, you'll never have any of the capabilities you have right now. So if you just installed it, enjoy it! You'll still be better off than 99% of us even after the new thing comes out.