Firstly, professional armies are recruited from the general population and are on average no better or worse than conscripts.
Secondly, the above comment completely sidesteps the moral aspects. Why should the burden of military service fall predominantly on the poor and the desperate? Why should decision makers be able to only send other people’s children to war?
There is no modern, professionalized army that wants conscripts. None. Conscripts are a liability, and a measure of last resort.
To your second, it’s far from just the downtrodden that fill the ranks of professional armies. In many countries, e.g. France (where I served), the upper classes of society (grande bourgeoisie and nobility) are over-represented in the ranks.
This isn’t true. The US and UK conscript armies of WW1 and WW2 were significantly healthier and better educated than the general population. Lots of people grew up in wretched poverty and had deficiency diseases or were malnourished or had parasites. Those people were rejected.
It is illegal for the US military to accept recruits with an ASBAB score of ten or below, roughly equivalent to IQ 83. The military is in some sense representative but it is not a random sample.