I don't know what
any of you are talking about. My [ð] and [θ] are very different, and are certainly not merging in any respect, although they aren't
perfect (/ð/ is usually something approaching [d̪ð] non-word-finally, but just barely, nowhere near a true affricate). You guys are freaks. They're in a semi-active-not-very-common-or-useful-I'm-probably-just-trying-to-preserve-something-derivational morphological pattern where nouns ending in [θ] change the [θ] to [ð] and lengthen the last vowel to change a noun to a verb, e.g. (please excuse my use of <[r]> here),
/riθ/

/rið/ <wreathe> 'to put a wreath on something'
/bæθ/

/bejð/ <bathe> 'to take a bath; to clean oneself'
/ʃiθ/

/ʃið/ <sheathe> 'to put into a sheath; to encase something, usually a weapon, by sliding it into a long, hollow, open-ended cylinder-like thing'
leading to unrealistic examples such as
/pæθ/

/pejð/ <pathe> 'to make a path'
/gɑθ/

/gɵwð/ <gothe> 'to make gothic'
Admittedly, this is far-fetched, but I'm a heartfelt supporter of it.