Oliver, I feel differently about both of your arguments:
1. Yes, if a U47 will ever hit $100,000, I think, spending 1/10 of the mic's value will be a reasonable price for a VF14.
There are plenty of examples from other areas of collectibles, like from the guitar world, where the price of an essential component will usually rise commensurate with the price of the item itself: if you want to find a zebra PAF humbucker for a $100,000.- Les Paul Standard guitar from 1959, you will pay now several thousand dollars for it. That PAF used to be $100.- when I worked at Don Wehr's Music City in SF in 1975, and the going rate for a '59 Les Paul was around $2,000.-
2. Of course, any tube can bust at any moment. But, as long as a thorough evaluation of the VF14 preceded its purchase, its chance for long-term survival in a U47 is better than any other microphone-specific tube I know of.
Your overarching question of "is it worth it?" is daily answered by those who pay the current market prices for vintage mics, as they did in the past, when the prices were much lower.
Who or what determines the value of a tool? The user who agrees to the price, in negotiation with the seller. That, then and now, is the true value of the item, not what we wish it to be (so that more of us could afford it, so that these fine tools don't end up unused in bank vaults, etc.)
Prices for vintage mics are not artificially 'driven up' as far as I can see: nobody is forced to pay more than he thinks the item is worth, not even by those 'collectors' who will never use the microphone in a studio. Even they will not pay more than what the microphone will yield in the open market.
The relationship between low supply and high demand and the resulting (steadily higher) price of valuable recording tools does not even change when the equivalent sonic value or emotional attraction is offered in a currently-made microphone (the KHE is one mic that proves my point.)