eventually the wires in the fold-over plug (which you should definitely not solder!) will wear but it does take a long time.
For comparison, think of how a scalextric car works; provided the track itself is nice and smooth, you can drive whole real (i.e. non-scale) kilometres without the copper braids wearing greatly, and those contacts are not only sliding but also conducting electricity at the same time, so might be prone to wear by arcing etc. This sort of distance would be the equivalent of many hundreds or perhaps even thousands of disconnections of a dynamo plug. IME corrosion is the real enemy, not wear. (BTW If you solder the wires you will lose the springiness and squashiness that makes a secure connection, plus you will run more of a risk that you will get a bad connection via fretting. Solder is not good for mechanical connections that are liable to move.)
If you are really worried about it, leave a little slack in the wires (a good idea anyway) so that they may be bared again and the connection remade if necessary.
The conventional way of mounting an accessory to the dynamo feed is to splice the wires into the wire from the dynamo, e.g. using solder and heat shrink, (pref of the type with built-in glue to help sealing). On most bikes this is done near the fork crown simply because it avoids running two wires down the fork leg. It is quite important to make sure that this connection is waterproof/corrosionproof, since water may run down the wires and into it otherwise; flux residues are liable to help corrode the wires if they get damp.
If you choose to use any form of in-line connector, this introduces many more connections that might go wrong, and this connector itself weighs a lot more than a length of wire does. This means that, unless well-restrained, it will tend to flap around (which may rattle and drive you nuts BTW) and more importantly it is exactly the kind of thing that frets the contacts inside and causes the wires to fatigue prematurely etc. For these reasons (amongst others) inline connectors are normally studiously avoided in wiring harnesses that are subject to vibration.
When I first saw the fold-over plug I had my doubts about it too. If I were going to revise it, I'd make it more like a connection scheme I know works, is reliable, and can easily be remade too. On my old dynohub (which went for about 25 years like this BTW, stored outside in the rain) I modified crimp terminals so that the wire receptacles became sockets (with open ends to drain rainwater) and mounted these semi-permanently to the hub. I used long blade type crimp connectors on the wires (of the sort that are normally used to connect to connections with grub screw-type terminals). The long blades were simply folded double and fettled until they were a snug fit in the receptacle.
The folded blades were slightly springy (by an amount that could be adjusted if required), and the square shape made contact on the corners inside the (round) receptacle, with a low insertion force. This was in such a way as simply pulling the blade out of the receptacle provided a good cleaning action. WIth a smear of waxoyl this connection was quite reliable and proof against corrosion. The idea was that the wires could be easily disconnected by hand, and in fact the wheel could drop out and the connections would automatically disconnect. I think I adjusted the spring action of the blades once a decade or so, and of course the wheel came out many times in that period. In the event of a crimp joint to the blade failing (which in fact they never did), it would be a simple question of baring the wire and temporarily stuffing it directly into the receptacle, perhaps with the unfolded blade (or something similar) as a wedge if necessary. I never had cause to remake the connections in that way, but I would suppose it would be easy enough to do.
So if I were to modify a modern hub generator's connections, I'd probably solder (open ended) cylindrical receptacles to the hub and use similar (eg folded blade) connections into them. But I have not bothered to do this, because the plug that I initially had doubts about turns out to be very serviceable.