The risk of sudden death with SVT and a normal heart (with normal heart I mean no structural heart disease and normal coronary arteries) is related to the WPW syndrome (accessory pathway between atria and ventricles) in combination with atrial fibrillation. The combination may be dangerous as atrial fibrillation (or flutter) can be conducted through the accessory pathway, causing a very rapid heart rhythm (300 bpm or more).
SVT itself is very rarely dangerous.
It's correct that some people may have "hidden" WPW (no preexcitation on EKG) with a pathway that may conduct impulses backwards. If so, PVCs are often the trigger of SVT rather than PACs. I don't know if such a pathway would be able to conduct impulses from atrial fibrillation?
Anyway, chance of sudden desth with SVT is very rare. But it may be a good idea to find out which SVT you have, and possibly get it treated? There are common variants of SVT which don't involve accessory pathways and where atrial fibrillation is no more dangerous than it would be without SVT.