In just couple of Years time, Makitas current 18v platform will become the Light Duty one, while XGT would be the Heavy Duty. Therefore no need for Makita to develop and release any new 18v HD batteries (although I would welcome 18v HD compact 3-6-8 Ah for the current LTX line at any time ) as the current LTX battery range are quite capable for the current range of tools. So it make sense if you want more than go XGT. If you see the other brands alternatives, Dewalt 60v, Hikoky Dual power, and batteries over 8 Ah these do not work in real life to an extend that it is worth using it. You either would end with one or the other. It is just too expensive and inconvenient to have such an mix and match. I am on 18v LTX for many Years now (Maintenance and Refurb business) and for the extra power when need I got my corded (mains) set and the colleagues around me on site are the same. Unless you work in a remote area without any power source all time , it might be the case you would need XGT tools, but really you can not relay on batteries only if you have to do a work day in and day out on regularly. It is inconvenient and expensive way, no employer or customer wish to pay that price. It is only the short time need, one off project, etc.
You are right that XGT will dampen the development of HPerformance 18v LTX tools BUT how much more performance, above the current 18v LTX Heavy Duty tools, do you looking for?