There was cheap test equipment before Rigol came along, and there is tons of product out there a lot cheaper than them now.
Yes, but most of them did perform vastly inferior.
They aren't selling because they are cheap. They are selling because they offer great bang for the buck.
They are selling because they offer decent performance at a low price.
And if you don't think Apple stuff is at *least* as buggy (hardware and software) as Rigol stuff, you haven't been paying attention. Bendgate, antennagate, batterygate, mapsgate and more. I'm not bashing Apple, I'm just saying shit happens for every manufacturer.
I never said Apple was free from problems (they certainly aren't), but especially their software is vastly more complex than Rigol's firmware, and while iOS regularly comes with some annoying bugs it's not half as bad as when Rigol produces a new product with a new firmware. So yes, iphone development requires much more resources than developing some low end scope like a Rigol DS1kz or DS2k.
And nobody out there is pushing the envelope of price/features like Rigol. They've had several home runs in a row (DS2072, DP832, DG4062, DS1054Z, arguably others).
Rigol was the first to get price/features right but isn't the only one. Siglent offers mostly similar kit in this class (and for some things like low end AWGs even has a much more attractive portfolio than Rigol), aside from the DS1kz (and I'll guess we'll see some competition there, soon).
This still doesn't change the fact that nothing in Rigol's products is really ground breaking. They are certainly good at getting the balance between price, quality impression and features right, and as a Chinese manufacturer they can produce at very low cost. But meeting a price target isn't as complex as developing technology further, which Apple has done in many areas (i.e. manufacturing processes, materials etc). Everything in a Rigol product aside from PCB and case are cheap standard COTS parts, and I'd be surprised if there are more than say 5 people working on the firmware for all their products.
Rigol will come up with new products when they feel the need for it, i.e. when there are more similar products and sales figures will drop. Until then they'll just sell on what they have now.