In Keilim 26:8, we read about how intention can in some cases bring about susceptibility to tumah.
עוֹרוֹת בַּעַל הַבַּיִת, מַחֲשָׁבָה מְטַמֵּאתָן. וְשֶׁל עַבְּדָן, אֵין מַחֲשָׁבָה מְטַמֵּאתָן.
The hides of a householder become susceptible to uncleanness by intention, but those that belong to a tanner do not become susceptible by mere intention. [from Sefaria]
Bartenura explains that we suspect that the tanner will change their mind and thus the tanner's intention is not considered sufficient to induce susceptibility to tumah.
However, in Keilim 25:9, we see that intention cannot override intention.
כָּל הַכֵּלִים יוֹרְדִין לִידֵי טֻמְאָתָן בְּמַחֲשָׁבָה, וְאֵינָן עוֹלִים מִידֵי טֻמְאָתָן אֶלָּא בְשִׁנּוּי מַעֲשֶׂה, שֶׁהַמַּעֲשֶׂה מְבַטֵּל מִיַּד הַמַּעֲשֶׂה וּמִיַּד מַחֲשָׁבָה, וּמַחֲשָׁבָה אֵינָהּ מְבַטֶּלֶת לֹא מִיַּד מַעֲשֶׂה וְלֹא מִיַּד מַחֲשָׁבָה:
All vessels become susceptible to uncleanness by intention, but they cannot be rendered insusceptible except by a change-effecting act, for an act annuls an earlier act as well as an earlier intention, but an intention annuls neither an earlier act nor an earlier intention.
For example (as given in Kehati), if a person decided to use an animal's ring for a person, then it becomes susceptible to tumah, and if that person subsequently changes their mind, it doesn't lose its susceptibility.
Why don't we apply the principle that intention doesn't annul an earlier intention for a tanner, and say that their intention is sufficient to induce susceptibility to tumah? If the object in question will only ever be susceptible d'Rabbanan, I case see why the Rabbis might have made a distinction in this case (due to the perceived fickle nature of the tanner's intention); but when the object will become susceptible from Torah law, how can we be lenient here due to a doubt about the tanner's true intention?