That can be true, but I don't think lower album sales are the most important reason why they don't release studio albums as often as they did in the past. Most of the times, the number of studio releases per period of time were specified in the band's contract with the record label. In the 60s, bands were forced to release two albums per year! Now this is more flexible, since a business model in which you want to have "the next big thing" simply doesn't work in a market as diverse as the one we have today. Many people blame internet piracy for the downfall of record sales, but I think it's more based around the fact that there are way more bands who can put out records every 3 or 4 years without the fear of falling into obscurity, and also because the music market isn't restricted to the UK and USA anymore. There's South America and Asia now, and not only Brazil and Japan. And as tours get longer, there's more time between original studio releases.