(BTW, you might disagree that we set such assumption. I think TZ database does not prohibit a same abbreviation used for standard and daylight at a time, although it is unlikely.)
Not only does tzdb not prohibit it, that feature was long used in Australian timestamps, as it matched more-traditional Australian practice. One can still follow this more-traditional practice by using POSIX TZ settings like TZ='EST-10EST,M10.1.0,M4.1.0/3' which uses "EST" for both Eastern Standard Time and Eastern Summer Time. If CLDR assumes that names or abbreviations must be unique, that's a problem that should get fixed somehow.
In this example, if "EST" cannot distinguish between standard time and summer time, then we don't use the abbreviations. The goal of CLDR is to provide names that people in a locale can reasonably understand - and standard/daylight names requires that people can distinguish one from another. In this case, "EST" can be "generic" name in CLDR. The concept of "generic" name is not in TZ db database. Anyway, CLDR tend to exclude many short names, because these abbreviations are not understood by people out of regions.
If TZ database project insists to make the change, CLDR project has to decide whether we should swap standard/dalight name in CLDR, and if we do, when is the right time
I'm proposing a patch so that the CLDR project doesn't need to make such an abrupt swap. CLDR can have a transition period as long as you like, during which CLDR will work with both current and proposed tzdb. [attachment "cldr.diff" deleted by Yoshito Umaoka/Westford/IBM]
I personally think we don't want to introduce artificial names introduced here. If "Irish Standard Time" is the official term, also recognized by people in Ireland, specifying time in summer there, CLDR should not change it just for this purpose. -Yoshito