In the culture reflected in Sanskrit literature, dhanyavaadaH itself is seldom to be met with. The mature usage seems to be dhanyo'smi, dhanyo'ham, anugriheeto'smi etc. When we imported the British token of politeness 'thank you' (offered for even such things as a waiter serving a glass of water), dhanyavaadaaH must have become popular. Now America is the global trend-setter, so every thank you must be welcomed; what better than svaagatam to do so? If we could abandon the dhoti in favour of pantaloons, start wearing shirts and such, there seems to be nothing wrong with either thanking or welcoming such thanks - as long as the speaker intends to do so. However, another question arises as to why Sanskrit must be put into service when English words would do? There are certain situations like the samskrita sambhaaShaNa shibiras of Samskrita Bharati, where usage of Sanskrit is mandatory; but in other situations. ShiShTaprayoga seems to be to use English for such transactions. At the least such a practice demands less from the listeners because the Sanskrit dhanyatva seems to be weightier than the English thankfulness, the way things stand in present times.
budhajanavidheyaH
N. Siva Senani