Have you looked at the Molex MTA system?
Ah, sorry, I wasn’t clear—I think I meant to say more of a crimp tool system—I want to buy one really nice tool now, for which I can get dies for potentially anything I want to do as a hobbyist in the future. In fact the proximate cause of thinking about this now is needing to do a few dozen butt splice connectors. I anticipate I’ll need to do other things in the future though, and I don’t want to end up acquiring half a dozen sub-par hand tools.
Whether this goal is achievable depends entirely on what kinds of connectors you end up using. For the most part, interchangeable-die tools are for larger terminals, not the small (and ever smaller!) ones we use in low voltage electronics.
You see this issue in the Knipex in your original post, you see it in other high quality interchangeable-die tools like
Pressmaster MCT, and, frankly, basically every such system I’ve seen so far. They invariably seem designed for electricians, not for electronics work.
In fact, the Knipex system is probably already the “best” such thing, and even so, D-sub are basically the smallest it supports.
Even among the Chinese tools, where parts are often available, you can find the dies for most of the tools — but not for the smallest contacts (SN-01, for example). Those are only ever sold assembled into the tool.
So while I totally understand your desire (and frankly, wish it existed!), I don’t think it’s a realistic goal. If you want good crimps, you will eventually end up collecting expensive tools. Especially with small contacts, the locator (positioner) becomes more and more important, because positioning the connector accurately into the die simultaneously becomes more important, but harder to do, as the contacts get smaller. Some contacts, like genuine Mini-PV, are surprisingly difficult to crimp by handholding the contact. Cheap tools don’t have any locators at all, which makes crimping slower and much more error-prone. The downside to locators is that they make the crimp tool
extremely specific. I am convinced that many of the dedicated tools sold by the connector makers actually use the same dies, just with a different locator attached.
Some advice: find some favorite connector types and try to use those as much as possible, with as optimal tools as possible.* When you can’t, choose connectors where you can buy precrimped leads — for something you’re only doing once, it’ll be cheaper to pay (for example) $2 each for 20 precrimped leads than to spend $500 on the tool, plus 40x $0.30 for the contacts (plus an extra 10 for practice), plus the wire, plus your time — or to spend 4 x $25 trying to find a cheap tool that works, plus dozens of dozens of wasted contacts for experimentation, plus hours and hours of research to find the tools to begin with and try to make them work.
*For example, I use JST XH a lot, and have considered spending the $300 to get the real JST tool, because it’s so much easier to use.