From listening to Seiler on FastTalk podcast 54 he was reluctant to give HR recommendations if you plan to take a polarized approach. That is one of the reasons I see no advantage to ignoring FTP, and every advantage to tracking FTP and max aerobic power and vo2max (along with some other metrics that use HR and Power). So I’d rather keep % FTP and continue using HR as secondary metric.
Put another way, you aren’t going to increase stroke volume and mitochondrial density because your HR is low (I get that sitting at a desk!) – you simply need a lot of volume at a slightly elevated yet aerobic breathing rate along with a lot of muscle contractions. Do that over enough time and it will sprinkle magic vo2max fairy dust on our adaptations… at least for those of us not gifted with huge vo2max or a 5-10+ years of experience.