I have been flying Alaska Airlines regularly since 1995 - had elite status almost every year. Traveling for business, I generally arrange my schedule so that there is a backup flight available in case something happens with the first flight - I generally can't afford to miss an appointment. That also leaves me in a situation in which I can easily take bumps if offered, and I readily try for them.
Up until about 2005 I could count on getting at least two bump vouchers per year; one year I collected five. Since 2005, though, they have greatly improved the accuracy of their pricing and scheduling algorithms. In the last five or six years, I've only collected two bump vouchers, and not for lack of trying. Meanwhile the overall occupancy factor of the flights has also increased greatly. So even though the planes are fuller, the frequency with which they offer bumps has declined dramatically.
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So, I hope that offers some comfort or assurance.
If you are at the Alaska web site also check to see how many ticket options there are for the flight you are interested in. There are usually three options for coach, "Lowest", "Value", and "Refundable". As a flight fills up the "Lowest" fare offering disappears an option, followed by "Value". So if the only pricing option you see for coach is "Refundable", then that usually means the flight is getting full.