This is what happens to supposedly-stable lens collections: a period of stability and satisfaction ends with a deal that's just too good to pass up. Whether or not it really was too good the transaction happens - and one deals with the aftershocks. If it wasn't all that good after all it moves on; if it's really good though, other lenses are re-evaluated for either quality (not as good?) or focal length (this lens FL of x makes an adjacent-FL lens or two expendable) or some other quality (why do I now own so many macro primes?).
The Sigma 50mm f/2.8 macro seemed my ideal 'last lens' that fit perfectly into the slot between 28 and the 70-150 zoom. It has AF, it's reasonably fast (both focus and aperture) and 1:1 macro. But then came the 100mm f/3.5 macro, which is pretty nearly as good at all those things in a longer yet lighter form. Yes it's just 1:2 without the adapter element, but I seem to be OK with that 90% of the time*. If it's so good at 100mm I won't carry the 70-150 - but then I have 28 50 and 100 - two of which are macros? Might as well lighten the load with a manual 50 to go with the all-manual 28! Yes that makes sense, and here's a 50/1.7 SMC-A for aperture control on the camera, how nice. A few chats and clicks and it's on its way.
Then shock #2 hits in the form of a bargain used DA40 Limited pancake lens. One has to see this lens to understand what 'pancake' means in the lens business - and it seems one must try a Limited lens to know what that means! This will be my first, and seldom does one stop at just one. The 50/1.7 has not arrived yet but it seems that its place has been stolen away: here is a tiny, sharp and fast-AF gem .. and 'tis said that the DA40 works with film or a future full-frame digital Pentax, so both my 50s are matched even there!
Now far too many dominoes are near to falling - time for 'damage' control, meaning a few choice lenses must go to feed the new beasts.
In any case, the journey continues for "my ideal kit".
I'm fully aware that it's a myth yet I cannot help but seek it...
* hey, that 49mm macro adapter fits on Limited primes!