The RCC believes that they know what they mean, but the Orthodox prefer the original wording because you don't have to add a disclaimer to explain the extra part.
Are you really sure they know what they mean?
That's a core problem with the filioque--it's ambiguity.
There are plenty of Fathers (Western *and* Eastern) who have used the term 'through', and while we would still object to its addition to the Creed (which in its original form is simply quoting Christ, the most authoritative possible voice), Orthodox have no problem with 'through the Son' as a theological opinion. And for the first millenium this was largely how the West defined the 'filioque' which was why it remained a minor issue and not one to divide the churches.
But since the filioque doesn't actually say 'through', it could mean something closer to the diagram above. And this ambiguity was something that concerned multiple Eastern Fathers.
In 1054, when the filioque became an important part of the schism between Rome and Constantinople, what the West meant by it was still ambiguous and it was not the filioque itself which was the cause of the final rift but the West's attempt to force it on the East (I don't have the native familiarity with either language to have a personal opinion, but apparently in Greek
, the filioque is much less ambiguous and much more clearly something like the diagram above as opposed to Latin). Then in 1245, Rome made the problem much worse by finally concilliarly defining the filioque--and what they defined is definitely not "through". I don't think the above diagram is a particularly good representation of what they defined either, as the definition of Lyon emphasizes a 'single spiration' but both the Lyonine definition and the above diagram are completely unacceptable to Orthodox not merely as an issue of process (i.e., Rome's unilateral introduction of the term) but as an issue of faith.
As noted, Roman Catholics under the last two popes have seemed to go back to the first millennium ambiguity/"through", but it's unclear how they can do so without jettisoning Lyons (and Florence) as authoritative councils (which would have far-reaching implications to claims of Papal authority well beyond simply its impact on the filioque debate).