Seeks to explain the suppletion of the diminutive suffix usually reconstructed as *-inën ~ *-iccë-.
What if **ici and **pici were originally not inflected in any way? The former could have developed into a particle and the latter was clearly an adjective. When Proto-Finnic developed congruence, they now needed to inflect these words. However, the *-c- made the inflection ambiguous; should the inflection proceed as a t-stem, as with the readily inflectable nominals like *käci, *veci, etc. or not (cf. *lapci)?
This ambiguity was avoided by adding a suffix to both, which shifted the inflection to after that suffix, allowing the original stem to remain invariable. The -t- could have been (correctly) retained in *pitkä (not **pickä) due to the comparative and superlative forms displaying the consonant.
The diminutive suffix for nouns would have been inflected already, as it was part of the noun, not the adjective. However, it would have remained as the only case where *-ci inflected with a *-c- rather than a *-t-, and the suffix was probably not used often enough in the nominative for a system like this to be tenable. This meant that there was pressure to replace the nominative form of this suffix with something else.
Now let us posit an adjectival suffix *-ne(n) that was originally invariable (no congruence). Adjectives and diminutives are closely tied; we already make this observation with the existing suffix, as it is used both for adjectives and diminutives, and there is also *-i used both for diminutives and as an attribute-linking suffix. For these reasons, the nominative of the original diminutive *-ci would have been replaced with this adjective suffix, and then the inflection of the diminutive suffix applied to adjectives as well when congruence developed.
If the adjective suffix was originally *-nen, this would have been further encouraged by the difficulty of inflecting that regularly.