Purpose of these resistors in the input of in-amp
You need to provide a path for the input bias current. 

If you want higher input impedance you can use an in-amp with lower input bias current (and higher resistors) or you can live with more offset voltage or use a bootstrapped arrangement. Or use lower gain and AC couple the first stage to another stage. 

In some cases you may be able to DC-couple the input stage. 

There are lots of possibilities, you need to define the requirements for differential and common-mode input impedance.