The are approximately infinite ways to solve this “problem”, starting with how you even define it. If it’s unsustainable at current levels, what does sustainable mean? Is the time period this year, fifty years, a thousand? What’s the gap? Does it include administration costs, or just the payments?

Once you define what the question is, you can start to answer it. We could increase immigration (very good point btw). We could raise taxes (if so, which tax? Corporation? National insurance? If so - employer or employee contributions?). We could raise interest rates. We could remove the triple lock. We could just murder everyone over 75. We could do a mix of everything - raise taxes a bit and only murder the over 80s.

How you answer the question depends a bit on facts and a lot on ideology. The fact that this article takes one potential solution and declares it THE ONLY solution tells you a lot about their ideology.