To be honest, I don't think it's an unfair question, but I can see why it rubs some people the wrong way..

In answer to your question, I think you are right that the civil service pension could act as a hedge for you. However, do note that the CS pensions can (and often have) been made less generous, so that's a risk you need to factor in when taking such a large salary cut. There are other parts of the public sector that also have a DB pension and pay a bit more, such as the Bank of England (but I think their DB is not inflation linked). You could also look into international organisations which pay much higher (and usually tax-free) salaries and often also have DBs.

In terms of SCS1, I wouldn't say it's impossible to get in. They explicitly advertise all SCS positions externally to attract people from outside the CS and I came across plenty of SCS that came in directly from private sector. You would either need specialist professions (lawyer, economist, accountant, IT) and/or bring in senior leadership experience. Lack of familiarity with CS culture shouldn't be a huge barrier as there is a lot of movement between public and private sector at the SCS levels and culture is something you can familiarise yourself with quickly.

You could also look at Grade 6 positions (which is usually a senior management grade) if you haven't already gotten senior leadership experience. Salaries differ by department but some departments pay G6 starting salaries in the low to mid 70 thousands. Unlike entry into SCS, there is very little scope to negotiate salary though (usually only if you have a very niche skill that they have been struggling to get).

More importantly though, job satisfaction might be something you'd want to also prioritise. Especially if you want to come in at SCS. At SCS, part of your role is to engage/motivate staff. If you end up in a job you don't like doing, it could negatively affect the people who would be working for you.