it means that quarantine is now half what it was (with vaccine). This is a great incentive. The UK for example, has already given 31 million adults at least 1x vaccine.
as Ron said- you could still encounter covid between getting tested and arriving in Thailand (Thais on the same flight are not required to take any test in order to return to their country).
results have so far shown in the uk that the vaccine actually lowers the transmission rate as well as providing individual protection.
Your sweeping generalisation anticipates that one can get infected and then redistribute the pathogen instantly- like a game of tag. This is false. The virus has to take to the subject and reproduce at number in order to be passed on. This takes days. The majority of people vaccinated will have an immune system capable of fending off the virus enough to stop a patient passing it on.