What happens if you have attrition in your customer base over time. An easy way to extend the model to take this into consideration would be to add a variable to model Attrition Rate as a percentage of the entire installed base at each cycle, and simply subract this from the total population at each cycle. This topic is nicely covered in this blog post by Andrew Chen: Is your website a leaky bucket? 4 scenarios for user retention.
I wouldn't think of a high yield savings account as a source of passive income but your savings should be getting something (less like Seinfeld syndication residuals and more like a commercial jingle residuals!). It won't make you rich but it's nice if your baseline, risk-free rate of return on cash is 1% or more. The best high yield savings accounts (or money market accounts) offer higher interest rate and there is absolutely no risk. CIT Bank currently leads the pack with the highest interest rate.
A truly viral product emerged from targeting a truly viral problem in the digital age, known as attention deficit disorder. Allowing people globally to channel their nervousness into an entertaining handheld device has allowed for the viral spread of Fidget Spinners. The products modest beginnings spread virally through school children and later through to adults. We started seeing fidget spinners in social media, memes with fidget spinners, fidget spinners distracting people while crossing the street, and of course, fidget spinners in the impulse purchase section of your local supermarket. This little product achieved a viral marketing status through providing a ‘solution’ to a viral problem and bringing about a world full of fidgetty temptation.