It's weird, but I sort of do see Walker as an "establishment candidate". Reince Priebus was his campaign manager in WI for his election and the recall elections, and his big wins there are what propelled him to the head of the party. So by that alone, Walker is very closely tied to "the establishment". His manager and closest political confidant is now head of the GOP. How more "establishment" does it get than that?
And also, Walker has been in politics his whole life, right out of college. Staffer, State Assemblyman, Milwaukee County Executive, Governor... that's his resume. So admittedly the moniker "career politician" does fit. That's another signifier of being "establishment".
At the same time, from his time battling to maintain balanced budgets in Milwaukee County with a liberal dominated board, then as Governor with the union fight, and the hippie sit-in protests at the WI Capitol going on for YEARS, and enduring the years malicious John Doe investigations (WI's crappy version of a Grand Jury) Walker's been kicking butts and taking names, and is a darling of the Tea Party/grassroots types. So he is in this odd gray area where he's both of the GOP "establishment" and a Tea-Party'ish "outsider" at the same time.
On paper, other than Jeb being the "establishment choice", and presumably Priebus needing to be "neutral" on that, it should make Walker a shoo-in, but he has a somewhat milquetoast persona and delivery, his voice is a little high and nasal, and he has a lazy eye... Unfortunately, 15 seconds of Trump is more interesting to watch and listen to, than an hour of Walker. It is what it is...
And despite the incredible political turnabout in WI that he's spearheaded, he's not actually that aggressive, he hasn't really gone picking any of these fights that he's so famous for. His strength is more in not ever giving in and seeing it through when the fight comes to him. He's more reactive than proactive in this way. And without the very strong hard-charging WI legislature behind him, and instead the weaker moderate GOP House and Senate in D.C... legislatively at least, I think a lot of people might find him a disappointment after four years.
For instance, had the WI .gov and teachers unions made some very modest concessions to their healthcare and pension contributions, maybe 10%, just enough for Walker to balance the WI state budget, they'd still have their unions, and 90% of their tax-funded gravy-train intact. And the entire shitstorm that put him on the national political stage in the first place would never have happened. Also, he was completely disinterested in passing right-to-work in WI. The state Legislature and various conservative outlets had to drag him into it. He was also rather weak on a Indian Casino deal, that I've gone into detail on in other threads.
That said, I think he'd be a "good" president, especially if he managed to pick brilliant people for his cabinet, but I don't think he'll be able to deliver a "Wisconsin Miracle" for the rest of the United States. Especially not with this spineless GOP Congress.