Quite possibly. I've been reading up on John Lindsay, a progressive Republican turned Democrat. What if he's an archetype of the kind of Establishment GOPer we're dealing with? How do we break this losing cycle?
The big problem is that the Dems are the party of government, and the GOP is not. If we want government to change, shouldn't we invade the Dems instead of vote third party?
If we had fiscally tightfisted, socially conservative Democrats in the bureaucracy and the judiciary, this country would be a lot better off and the GOP wouldn't dare play with the Larry Craig Republicans. While a lot of us should still fight on in the GOP, perhaps those tempted to go third party should instead go "rogue Democrat" -- challenge Democratic state legislators in primaries in "safe" districts, be "the weird guy" at party meetings, become delegates to the county, state and even national assemblies, etc.
That bipartisan strategy is how America was subverted. Going Third Party just lets both the Dems and the GOP become even more nutty.
Your idea has appeal. I've thought for a while that for Pubs like Mike Huckabee, if they really are as pro-life as they claim, but agree more with Dems on economic and foreign policy, it would make more sense for them to join the Dems and work to make two pro-life parties, one socialist and the other free-market, which is the way it was until sometime around 1980 (even Ted Kennedy was once ostensibly pro-life!). Likewise for Pubs who are one-issue, portfolio-only Pubs.
Unfortunately, most people attracted to politics are opportunists, and take the path of least resistance to get power and attention, depending on the prevailing winds wherever they are when they first get into the game.
As far as real, complete constitutional conservatives going Dem, I couldn't do it, myself. Living a lie the way some RINOs do would be too much to take, and I don't see how you could go into the Dem party with your Gadsden flag flying and have any hope of making inroads there.
I think the answer is for constitutional conservatives to stand firm on the most important matters of substance, but not rule out any strategic option (any more than it makes sense to rule out either the literal or figurative nuclear option in war or politics). Also, it's important to force each candidate to prove he's acceptable to us, rather than letting them take conservative support as a given while they try to sell themselves to liberals and the mushy middle. Otherwise, we become as owned by RINOs as, say, blacks are owned by Dems.
When there's no acceptable candidate for one office, I give my support to conservatives running for other offices. Only the constitutionalist conservative agenda is worthy of support.