Water feed pipe diameter: is 1/2" OK?
I'm assuming when you say &quot;water feed&quot; that you mean your incoming water main from the water company or well.  If that's leaking then it needs to be replaced which I believe you already know.
I am also assuming that by proposing to &quot;bypass&quot; with 1/2&quot; PEX they are somehow going to put another parallel pipe in place or try to force the 1/2&quot; through the existing 3/4&quot;.
I'm not sure either of these plans are all that good and the main reason is that you'll now be stuck with a 1/2&quot; water service to your property and that's likely going to be deficient.  The 1/4&quot; difference sound slight but remember that the cross-sectional area of a pipe (i.e. the IMPORTANT part) varies with the square of the radius.  So the 1/2&quot; pipe has an area of 50% of the 3/4&quot;.
This comes from simple calculations using the typical ID of 3/4 and 1/2 PEX as 0.681&quot; and 0.485&quot; respectively.  Using the formula of A = pi * r^2 you can calculate the two as having cross-sectional areas of 0.36 sq-in and 0.18 sq-in.  That's 50% less and that's a LOT!
Let me suggest an alternative.  It's quite possible for a horizontal drilling contractor to drill in a new line and hit a 12x12&quot; hole in your slab.  Using that they can then pull in a new 3/4&quot; (or even larger) pipe and you won't be left with an inadequate water main.  They will likely need to cut such an opening in your slab and also will likely need to excavate a hole to get access to the water main.
Many plumbers work with these contractors but if not, you should be able to find one using Google or similar in your area.