Shouldn’t we use a tallit with the purple thread.
This is a delicate issue that many have addressed in recent years, and focuses around two questions: 1) Is there clear scientific proof that the techeiles used for the dye is the Torah-true form; 2) Given the answer to (1), is there enough justification to obligate wearing techeiles.
Many rabbis who look into the matter actually do become convinced, and some even begin to wear the techeiles. The general “excuse” for not looking into the sugya is that we don’t see that the gedolim did so, which is rather a weak answer to a good question.
The great majority of the observant community, and certainly the Chareidi community (including its leading rabbis), continues to refrain from wearing techeiles, but the numbers are increasing. Recently Rav Asher Weiss shlita published a lengthy Teshuvah explaining the reason why there is no obligation to wear the modern techeiles; see the second volume of Shut Minchas Asher. Some have written extensively on this, too – please see comments here.