I find the adjustment of LTV by 0.75 to be a smart shortcut. You could probably spend hours and hours of calculating just to reach an average of 0.8342 or something, though the exact number wouldn't make much difference to your CAC spending.
One important thing LTV misses is payback period. E.g. due to very low churn your LTV could potentially be huge but the value would take many years to realise, depending on your cash reserves CAC spending well below LTV still may not be economical if it takes too long to realise the ROI.
Agree with you here. For my previous start-up I used to calculate LTV only based on people that churned. Knowing your LTV is much higher can only be great, at least as long as you can work with a CAC/LTV value.