It doesn't work like that alas - the AZ thrombosis is a pretty serious condition and the best theory we have right now suggests it can't be just treated with an aspirin.
That, together with the fact that this group has minimal risk from the disease means it’s concerning. You don’t want to give someone a vaccine with risks of the same order as the disease.
Giving AZ to 60+ is still almost no concern even with this rate of adverse effects because they show a lower thrombosis risk and of course a massively higher risk of severe illness or death from the disease.
Countries with a mix of AZ and mRNA vaccines like many EU countries can probably use AZ for 60+ and since AZ underdelivers, the rest can take mRNA. Countries that have only or mostly AZ are in a more difficult situation.