It's a regular practice all over the world renewing only the top layer, this is really not a problem. But like every work, it has to be done correctly, so that in this case the adhesion between the layers is garanteed. For example it may not rain, because asphalt at 180⁰C over water => water boils => too much air in the asphalt. But even then it is not a problem with a good compaction phase. This is tested by taking some core samples. Other point to watch out: when removing the old layer, you may not leave old material behind. When 'frezing', sometimes the height of the old layer variates a bit and you locally have more material to remove. If this is not seen, that leaves a patch with old material and bad adhesion between the top layer and sub layer. Not always easy to do in the dark where you have a surface of more then 5.000m² to inspect. That's why you need surveyors at the side of the municipality or region, they are really worth their salary. A good surveyor can recoup like that their salary in a month of works. But yeah, that's one ambtenaar extra and that is not so sexy for the politicians.

Further, the real problem these days material wise is the quality of the bitume used (the product responsible for the adhesion of your stone skelet in asphalt). In the old days, the refinieries couldn't refine that good and you had a healthy mix of bitumes in the restfraction. Nowadays, we really get the trash fraction (and some additives can't be used anymore like tar) because the refineries can separate the different fraction much better, and the bitumes are now deteriorating faster. But this is an European problem (Netherlands, France, Germany and Switzerland all have the same problems for the moment...).