It's hard to say with any certainty, but I'd bet you'd mostly be fine, especially if you have flexibility as far as which days you can go. Fridays might be iffy, for example. A lot of days will likely fill up as passes are selling, too.
Having said that - they tend to replenish supply periodically - so if you miss out on a day, you can typically catch it later. (For example, some of the days around Halloween have been sold out for a while, but became available again a couple of days ago.) If you're local enough that you can react on a moment's notice, you can almost always grab reservations the night before, too - a lot of people drop last minute. It's not as much of a challenge as it might seem.
If it were me, I would consider buying the Inspire anyway, even in the unlikely event that you can't find a reservation in December. The year doesn't start until enter a park the first time, so you'd still get the full term out of it.