It depends to a large degree on scheduling, right? If ND waters down its schedule going forward, assuming it can handle the NIU's of the world so that the strategy actually works -- BY NO MEANS, A SURE THING -- then that might be its best option. If it takes on a tougher schedule, its odds of reaching naturally drop.
As for Big Ten membership, I'm not at all convinced that as you put it, "ND could easily finish in the top three and be virtually assured of a playoff berth." I mean, OSU, UM, USC, PSU, Oregon and Washington could plausibly all have equal or better teams than ND in a given year, not to mention Iowa, Wisconsin and UCLA in other years. Plus, Maryland and Nebraska can be tough and Purdue can't stay down forever.
Even though I'd like to see ND join the Big Ten, I believe it could cobble together a schedule friendly enough to give it a better chance of making the playoffs as an INDEPENDENT. More control. But then we're back to that OTHER PROBLEM of Big Ten and/or SEC teams potentially FREEZING ND OUT by not scheduling it.
What I'm ACTUALLY for most is the MOST AUTHENTIC REALITY TEST AVAILABLE. And for me, that would involve ND joining the Big Ten, come what may. Because ND would then be forced to compete with the toughest likely PEER GROUP possible and by doing that, SEE WHERE IT REALLY STANDS. In other words, could it take the heat in the Big Ten on a WEEK TO WEEK basis?
Once it saw where it stood, ND would either have to put up -- GO ALL IN FOR FOOTBALL which is highly unlikely -- or shut up and potentially live with a lot of 9-3 and 8-4 seasons, those being its average in GOOD YEARS.
Bottom line: KEEP THE SHAMROCK, LOSE THE ASTERISK.