Mary has a €500k home with a €300k mortgage and a pension fund with €500k invested in equities.
She inherits €300k
She should pay down her mortgage with that.
However, a lot of people compartmentalise their home and mortgage - and look at their other investments completely separately.
If Mary does this, she has €500k in equities so she diversifies by buying an RIP for €300k.
She thinks she is reducing risk, but she is increasing her risk compared to simply paying off her mortgage.
I find this "balance sheet" approach to wealth overly rigid and lacking nuance i.e. Netting off Assets and Liabilities as proxy for security.
The most valuable financial asset you have (when you have limited capital) is your ability to generate a income. Your day to day security is really, can you generate more income than your expenses each day, on average. Rather than the value of assets and liabilities rising/falling in value. Cashflow is the name of the game. Diverse and secure sources of cashflow is real financial security i.e. being able to meet all your outgoings in a variety of economic conditions, such as if you loose your job.
And when looking at cashflows, the fungibility of assets is important. I can sell part of a paper asset, but can only sell a whole unit of a hard asset.
If I have a salary, say I use it to pay mortgage and non-mortgage expenses. If I pay off my mortgage, it eliminates only part of my expenses and that capital is gone (from a utility point of view). Loosing my job - I still need to meet my non-mortgage expenses. If I have mortgage and non-mortgage expenses and loose my job, but invest the available capital, I still have a large financial asset from which I can cover my outgoings. The difference between the two positions is the duration with which each position can be sustained before ruin occurs. The longer I can sustain the position, the greater the probability I can recover (therefore the greater the security).
With this way of thinking, obviously, discipline and financial literacy are important while also balancing ones approach. The extreme position is paying off a mortgage completely, removing someone's recourse to a property, which is great. Its not as clear cut as paying off 10,20k at a time i.e. reducing your mortgage expenses by €50/100 per month. If you lose your job - you're still in the position of trying to cover €1900 per month rather than €2000, but if you retain the €20k, you can sustain yourself for 10 months.