Personally i think unsold token must be burned in order to keep the price stable
Burning tokens or cryptocurrency most of the time is just a way to bring an incentive for investors in the long run, as that would reduce the circulating supply and it is not just held by the team and for publicity these days, and to assure the investors, it has become a common practice these days.
What is most important in the long run is simply the demand, even though a lot of projects tend to buy back and burn, the essence of the burn would just have been to reduce the circulating supply and make it grow in value quickly based on the level of demand for it. I personally prefer unsold tokens to be burned anyway.
I also prefer if unsold tokens are burned too, of course price of the token will be competitive and dev can develop with the funds already reached. besides that it will certainly be a transparency for investors to continue to support a project