Generally agree with this, however… some additional food for thought as everyone's appetite for risk vs reward is different -:
If you buy an Economy "Sale" fare, then put simply, you are ineligible for an upgrade request. If you still wanted to go down the (potential) upgrade path, you would need to look at an Economy "Saver" fare (or higher), which of course means additional cost. Importantly, you’d need to decide whether you're comfortable paying that extra cost and still having your upgrade request rejected because in the event that it is rejected, it won’t matter whether you paid for a Sale fare or a Saver fare - you’re still stuck in Economy and the product/service is the same. However, if the price difference isn't significant AND you can stomach the scenario described above AND you're still willing to roll the dice, then buying a Saver fare (or higher) remains an option.
By way of example, I rolled the dice last year for upgrading myself and a travel companion from Y (i.e. Economy) to J (i.e. Business) on a SYD>HND flight (which is a very popular route), but that was knowing that I'd be Platinum by the time I flew and that would most likely bump me much higher up in the upgrade queue. On that particular route, there was no Y+ (i.e. Premium Economy) on which to fall back, so in effect it was an all or nothing gamble. I got lucky on that occasion, but can guarantee that if I was Bronze (or Silver) there was absolutely no way that upgrade was going to be successful given the flight loading. Specific to the idea of a Bronze FF scoring an upgrade to J on a long haul to London? Personally, I think it's highly unlikely to succeed and would therefore agree with [USER=34147]@jpp42[/USER] that paying the extra is a bit of a fool's errand. But if the price is right and your appetite for risk allows, then buying the higher priced ticket and shooting for J (knowing there's the more realistic possibility of scoring Y+ instead) is always an option.
Caveat emptor applies as always 
