I am not sure that the cheaper cost of debt is the main thrust....

I am not sure that the cheaper cost of debt is the main thrust. Nice to have but these are my thoughts:

They could have just done plain vanilla convertible bonds and had conversion rights into shares ...

I think it's in the comments: "Following the Offering, WHSP will continue to have low gearing and diversified sources of funding"

I think the real reason is that the current dividend is too high for the current share price if you are going to do the deals like REG - There the 4c dividend (which I think is unsustainable anyway) including franking generates only 2.7%. So issuing SOL shares (which including franking pay around 3%) for the acquisition would not help as you would have to partially fund the dividend from other income and in fact in the short term it would reduce earnings. So rescue situations or restructure would cost unless it was debt so here is a model that is flexible. You have a listed instrument that is debt but also carries a conversion. As the trigger for the conversion price changes appear to be a SOL dividend above 62c which is an increase of 2 cents it's implied that this will occur into the future. I assume they can add more to the initial float at a later stage. This does not look like a one-off. So they will borrow at rates around 0.5% - guestimate... That is also tax-deductible and thus they have a source of cheap funding with a 5-year horizon.

The investor gets a tradeable and convertible product.

Then in fact BKW also benefits because it gets paid for lending 3 million SOL shares into the scheme that gives liquidity for those converting.

They have gone to a lot of trouble and they can control their debt maturity and still make investments without too much dilution whilst they grow the investments or restructure them.

All in all, I think it's very clever. Certainly not something to send the share price down in my opinion. More of the courage they had when they ventured into TPG or some of what made NHC a large contributor. I have been concerned for next years dividend - not the franking but the cashflow these sort of deals will make it easier to see that being maintained.