Over time strings stretch, albeit slowly. Slow enough we don't notice it. The drawlength gets longer and the poundage goes up from the original setting. When you replace the string the bow is back to factory specs. Shorter draw mweans less weight. Less weight and drawlength means less speed. This is simple physics.

Yes, the new one will stretch a bit, but newer string materials don't stretch as much as older ones. Thus, the bow holds it's tune better for a longer period of time. If you absolutely have to shoot 70# then crank the limbs in a bit (if they are not bottomed out). There aren't any deer on this planet that know whether they were hit with an arrow doing 242 or 231fps. If you have to have more speed then shoot a lighter arrow and/or clean off any unnecessary string accessories.