OK, I'm going to assume you're talking about the drum unit, DR-420 on those. This is actually what transfers the toner to the paper, the fuser is a separate unit that's likely got a programmed "hey, replace this" at 100,000 pages. On a lot of printers with more expensive toner this transfer roller is actually built into the toner cartridge, which is why those toners are more expensive.
The printer may complain when the drum hits its page count, but it should keep printing with no real issues - you can keep using that drum until/unless you start to see a decline in print quality.
List on the DR420 is a little over $100, so yeah, more than a HL-2240 typically costs but even at the high price I see of $110 that means you got a hell of a deal on the printer at ~$40ish. I'll also note that third-party drum units run $18-25.
In any case, most people who get that model printer are never going to come close to cranking 12,000 pages through a $40 laser printer. In fact, if you've run that many pages through I kind of hope you're using cheaper aftermarket toner, because if not then you should've bought a heavier-duty printer with a lower cost per page for toner. When you're looking for a replacement, check how much the toner cartridges cost per page (price/pagecount), plus any drum replacements and whether it ships with a starter cartridge or a full toner cartridge. Still, at 1.8 cents per page for toner I'm pretty sure that Brother's at the low end of the small printer toner cost scale.