I'd re-examine your second statement. Most vintage speakers that I have actually seen the crossovers on - JBL, Infinity, EPI, KEF, ADS, Advent, KLH, Dynaco, and others - have electrolytic caps which fail after around 20-30 years. Some of those mentioned also have metalized polyprolyene as well. Had some newer Energy Veritas and IIRC, the XO were mostly high grade MPP, but may have had some 'lyics, too.
Some speakers have only film caps in the signal path, but may have electrolytics elsewhere in the XO.
When the XO caps fail, it typically chokes off the signal to the tweeters. Also, replacing an old electrolytic cap with an MPP of the same value usually produces a smoother, sweeter top end. MPPs are more expensive, though.
One other thing to consider is that the newer electrolytic capacitor manufacturing techniques are much much better than those of the vintage era, and that can be seen by the difference in cap size.