Next month NDS will be at ~93.4 million units. Seems the lead then will be bigger than ever before.
A month later NDS is at ~95 million units.
Month 54 NDS will cross 100 million, which is August 2021 for NSW.
Yes next month the gap will be ~8 million again, Switch will probably gain 500k-1mil the 2 months after that, then probably hold steady while they both go through non-holiday period.
DS is a tough nut to crack for sure, but month 50 is the end of its peak period, and Switch is just about keeping up with it (only 244k down past 12 months).
You mention DS will cross 100 million in month 54, but Switch won't be too far behind as it might pass 100m in November which is its month 57, and if it doesnt quite manage it in November it will blow past it in month 58 & probably be within 1m of DS again. If Switch can hold some momentum into 2022 then it might be able to mitigate DS's last big holiday (month 61&62 aka this time next year) so the gap never gets back to 8 million.