Doing resampling, how can we choose lower cutoff frequency for LPF filter?
One thing to consider is the existing spectral content of the signal being downsampled.   If there is no significant energy above 15kHz, then the anti-alias filtering requirements are certainly relaxed since it won't have to do much for a 30kHz output sampling rate.

So where to put the filter cutoff frequency also depends on assumptions about the spectral content of the input signal as well as the transition band rolloff characteristics of the filter.  For a 30kHz sampling rate the 3dB cutoff point of the filter may need to be much lower than 15kHz to prevent aliasing from 15.5kHz or 16kHz (for example).

Much of this depends on your tolerance for the level of aliasing and at what frequencies.