In my opinion, expansion campaigns are those that are not supported by investors. If you continue to participate until the end of the campaign, the rewards you receive will be very low and not worth the time you spend, I guess. Anyway, that is still your decision.
If they don't expand the bounty budget, it's best to stop and look for a better campaign. Campaign expansion is not bad, sometimes they need to expand it to make it easier for ICOs or IEOs to succeed.