The client may instead have no firm plans to sell any of the properties, instead a plan to keep an entire portfolio.  Even where debt still remains, enough time in the market can eventually result in a cash flow in retirement that is sufficient to pay all costs including loan interest and still provide a suitable income stream in retirement.   In some cases that cash flow is large enough to allow the investor to continue to borrow well into retirement and so continue to build their portfolio.

In this scenario, even though the client may have made considerable gains in the value of their properties, no capital gains tax will be payable as the gain will be unrealised.  Although this gain is not realised as actual cash, its existence can provide the capacity for additional leverage into more property.