I don't think you can predict the reliability of shutters without factoring in the age of the lenses that you're comparing. The original all-metal lenses might have been better out of the box, but... they're also now considerably older than the KLs and possibly well used. If an older lens craps out on you, you wouldn't even both to repair it, you'd simply replace it. The older RB lenses are a dime a dozen.
Anyway, as someone who has a lot of new and old RB lenses, I see no important difference in build quality between them. I am 100% pleased with the performance of the KLs, and they do offer very real optical advantages, to my eye. I don't concern myself with hypothetical disadvantages, when I have plenty of real advantages to enjoy e.g. better coatings, better colour neutrality.... and fresh warranties! And the peace of mind of knowing that the lens wasn't beaten within an inch of its life before I picked it up used.