Can a planar transformer replace a laminated transformer for 50Hz AC conversion?
It would be extremely difficult because of the number of turns required for the primary (hundreds). If you had an expensive board with many, many layers then maybe. 

To get a certain magnetizing current, the primary inductance required is proportional to the operating voltage and inversely proportional to the operating frequency. So for a frequency lower by a factor of 100kHz/50Hz, you need an inductance 2000 times higher, or about 45x as many turns (square root). So if a 10 turn primary would do at 100kHz, then a 450 turn primary is required at 50Hz. 

Mostly the available cores are ferrite, so they would not be very appropriate for mains frequency either. Laminations of silicon steel are normally used at mains frequencies. 

It may in fact be possible, but probably not economical.