The Peavey look may be dated, but that is not what is keeping down sales.
1 middle of the road sound
2 weight
3 perception of quality
In that order. That's why the majority of what it sells is beginner.
I'd say:
perception of quality (false), middle of the road sound, perception of weight (false).
The current Peavey stuff is no more heavy than any other equivalent stuff. As for the old stuff, part of the weight was because Peavey didn't use the cheapest lightest stuff - that's what made it tough. Also, guys who wanted a bullet proof and LOUD speaker like the BW had to deal with the weight - that's how you make a speaker like that.
Unfortunately, you actually have highlighted the problems for Peavey in that musicians are very conservative - read the number of guys "I bought a Peavey in 1976 and..." Peavey has updated its look regularly since the 80s. Their sound has improved (and is quite popular for high gain types of music) and the equipment is considerably lighter than it was.
If car buyers were buying based on their experience in 1976 there would be no Ford (Pinto), no Toyota (early rust-buckets)...
As for the majority being beginner - so is Fender's sales, so is Ibanez, Yamaha, even Gibson's main market is the under $1000 - and they make most of their money these days from the Epiphone line of which the best seller is (last I read an industry mag) the $200 bolt on les paul special.
The majority of sales to beginners comes from the fact that the majority of music equipment sales are to beginners. Add in that Peavey was indeed never a "boutique" brand and didn't court that market for the first 25-30 years of its existence it only follows that most of its sales would be low end.
But the Bandit is still one of the top 4 or 5 amps sales-wise of all time and wouldn't really qualify as a beginner's amp - they're loud, they're reliable and they're too expensive new for the vast majority of kids - they are working musicians' amps and they serve the purpose well. Their clean tone isn't as well regarded as Fender's and their distorition not as much as Marshall's but they do clean better than Marshall and distorition better than Fender (as a general rule) so, don't be too hard.
Again - I don't personally play or own ANY Peavey, but I recognize that that is a matter of preference (not bad tone, just not my tone) and I am, having been raised by a Fender-guy and worked in a Music store for much of my youth a gear snob and prefer the "prestige" brands of Ampeg and Fender. The only difference is that I know I'm a snob and that most of the audience (including the musicians) cannot hear the difference.