Based on the most recent studies I have read (or rather, mostly meta-analyses of recent studies I have read), this is what would happen:
- Strength-wise: at first, your 5x5 would be a significant intensity level which means that you would get stronger. As the reps get higher, the intensity would taper down to much lower level which means that it would yield less and less training effect.
- Hypertrophy-wise: (assuming the volume hypothesis) at first, your 5x5 would yield a low volume (5*5*your bodyweight). As you progress to more and more reps with the same weight, the volume would progressively increase as the intensity decreases so the hypertrophy effect should increase as well, until you become so strong that the load is negligible and would not yield any hypertrophy adaptation at all. (assuming the hard sets hypothesis) since you are continuously pushing your sets to the max (i.e. every set is hard), the muscle recruitment stays identical/maximal all along your training cycle and the hypertrophy effect is more or less constant. Even more, as your strength increases, your max reps approaches your actual muscle failure point but that's mitigated by the fact that your strength will grow slower and slower if you don't add additional weight.
Overall, it will work well up to a point and then stop working, as most things do in fitness.
My 2cts would be that it would work better in the other direction. Methodologies like block periodization make use of something called phase potentiation, which means that one phase of your training makes you better at attacking the following phase of your training. Traditionally, a hypertrophy phase (meaning lots of sets and reps, as you would have at the end of your training cycle) powers you through a strength phase (what you would have at the beginning of your training) (which powers you through a "power" or peaking phase depending on your sport).
