Hard to get a gauge on Grassley's motivation other than it has been his "job" for 40 years. Iowa is an interesting and odd state politically, with lots of pockets of hardcore libs (Iowa City, Des Moines proper discounting the 'burbs, and Cedar Rapids) and pretty much every college town. Rural Iowa and the suburbs are certainly conservative.
Wish that he'd step aside, but don't know if there is a great conservative to fill his spot. Assuming he does win re-election, he could step down in mid-term while Republican Governor Kim Reynolds could replace him.
Want all of the Crypt-Keeper lifelong Senators and Congressional House members to go away sooner than later. But you're correct, they seemingly crave power.
As I see it, if there isn't another "great conservative" to fill Grassley's spot, that's a direct result of someone who is not just elected to office, but embedded in office. Grassley does the state of Iowa a disservice by his being a professional politician -- nobody can beat him, so nobody tries.
The Founders believed that serving as a Representative or a Senator was a duty and not a career. Back in the day, they'd serve and then go back to their business of farming, lawyering, doctoring. They've continued to add perks and benefits that the average person would never see, and thus becoming a professional politician was something that the power-hungry and money-hungry sought. This is what we have now, on both sides of the aisle.
Disgusting.