Note that PBKDF1 is exactly that, vanilla chaining of the same hash function. And AFAIK is not believed to have known attacks, with the only drawback being the fixed output size.
I guess that the xor approach used in PBKDF2 is useful since you want to compute T1, T2, T3 ..., Tc that are multiple output blocks all starting from the same input, so this gives more information to the attacker and the schema is designed to avoid showing some "state" that is possible to more easily analyze. Not sure, but the point is, I don't think PBKDF1 is unsafe either, and it is just chaining.
I guess that the xor approach used in PBKDF2 is useful since you want to compute T1, T2, T3 ..., Tc that are multiple output blocks all starting from the same input, so this gives more information to the attacker and the schema is designed to avoid showing some "state" that is possible to more easily analyze. Not sure, but the point is, I don't think PBKDF1 is unsafe either, and it is just chaining.