Herbie, will probably answer on the technical side but from what I have read you should be able to run your 2015 NX for many years to come. Batteries, I believe, are at their most efficient when charging and discharging at the same time. Hybrids do that. The consensus of opinion would suggest that possibly age rather than mileage is more important. Prius taxis achieve Moon shot mileages on the original batteries. Very few posts have appeared from hybrid owners who have had major battery issues. And there is now a much cheaper fix through the firm Hybrid Battery Solutions. I would have no worries about keeping the car long term - other issues will finish the car before the battery.