As discussed here already, the number of occupied or open seats is not an indicator of the booking situation. Business class could be heavily overbooked and still 18 open seats could be shown a week before departure. Many pax do not or cannot pre-reserve seats. For example, if you are on a codeshare flight number, you cannot easily reserve seats on LX. Plus there are pax that book on short notice, change flight dates, upgrades from PE and Y etc.
In my experience, the likelihood for an upgrade with the minimum bid is small, as are your chances that upgrade prizes are cheaper at checkin.