I wonder if you could build in a small algorithm which undershoots efficiency to start, then slowly ramps it up over a few years (and maybe shrinking a reserve). This would give you the feeling of a stable usable range, and manage the battery decline for say 3 years, which would give people a good feeling. Then a gentle decline would be considered acceptable as the car would no longer be "new". It would also manage through most lease agreements.
I follow some of the Tesla boards, and the amount of screaming after the software change has been deafening (and as they are well healed lawsuits are following). Also, there are a growing number of Bolt drivers reporting 10% range drops and more after 2-3 years of driving. While I would put both at rates less than what most (now not all) leaf drivers experience, its not 0. Maybe 3% a year across it all. Our 13 leaf, which was at the center of the curve, lost about 20% after 6 years, 11 of 12 bars, but we only put on 6K miles a year (traded it in at 37K miles).
2019 S Plus (93.86% SOH) & 2019 SV Plus (93.35% SOH) Both Silver
2013 Leaf SV sold 2019 with 11 bars
100 Mile Club Member (Number 87)
Max Miles on 13 Leaf: 120 miles
Max Miles on 19 SV+: 242 Highway miles @ 4.5 miles/kWh