Seizing a fief (now) legally held by another dynasty, would exactly be the thing which would get Wenceslaus deposed from the Holy Roman Throne. It sends a very worrying signal to other vassals of the Holy Roman Empire.
Legally Ottokar II lost the Austrian possessions, when the Imperial Diet, not just Rudolf of Habsburg, placed him under the Imperial Ban. In the subsequent peace treaty Ottokar II was reinvested with Bohemia & Moravia, but Austria, Styria, Carinthia, Carniola and the Windic March reverted to the Imperial Crown.
When Rudolf invested his sons with Austria & Styria, this was confirmed by the Imperial Diet.
This would be a totally different situation, than the one where Ottokar II used the Great Interregnum and an unclear succession. Now Wenceslaus would deprive the legal holder of a fief from his possession.
Furthermore the only prince of the Empire, which would benefit from this is the king of Bohemia. Regardless whether the other princes like the Premyslids or the Habsburgs, it's better for them to keep Bohemia etc. and Austria etc. separate, unless it happens in a legal manner they can't do much about without undermining their own position, for instance by inheritance. IOTL Rudolf reinvested the Premyslids with Bohemia & Moravia, he managed to secure Austria & Styria for his house, but Carinthia & Carniola went to Meinhard of Görz-Tirol (Rudolf's ally); a division instead of a concentration of power.