I somehow doubt they are going to comply with your pleas include the sandbox in the free version. It isn’t only the sandbox, but what is left to differentiate the free from paid versions, avast free is already the strongest competition avast Pro has.
There has to be a difference and the programmers, etc. have to eat.
You have a very good free antivirus, which isn’t crippled like others, it has a very high percentage of the paid options already.
There is nothing to stop you using something like sandboxie with the avast free version.