There will be always some improvements or new things in new models, so even you buy possible upcoming 2019 version, something better is coming in the future. There is no end to this. Besides no one knows what or if anything is coming this year. This current model is already completely renewed last fall, so I don't expect them to do any dramatic changes to chassis which means they cannot change keyboard very much. There is no room for longer stroke keyboards in the current chassis unless you put in smaller battery. Sure they could do that since possible new cpu consume less power in light load but then again it probably has higher TDP than the current one - meaning that it will draw much more power under high load causing less battery run time, so combine that with possible smaller battery and you will have a lot of unhappy customers. Personally I don't see that happening but I've know to be a bad fortune teller, so you never know.
Also you realize Apple offers 4 year warranty for the current Air 2018 models, so that should give some piece of mind. Sure I'd hate to send mine to service but still better than nothing and probably you may get 4th gen keyboard so maybe it will be better.