If you are evaluating a project from the MSFT perspective you might want to use their cost of capital, but as an investor if I can go get 10% why would I want to use 7% as my rate?
In any case lets use your 7% you end up with 400bill which is still 50billion less than what it is trading at.
Lending and borrowing rates are closely tied together since they're literally just two sides of the same market. Global long term rates have been slowly trending lower for the last few years.
Over a long period of time that is what the market has returned. It is not guaranteed.One year it might be down 50% one year it might be 50%, but if you look at 30years S&P has returned about 7.5%. 10% is a just a round number makes math easier. You can plug in a different number my point its hard to get to 450 billion.
The S&P only had 500 companies starting in 1957 using that as the start date you get: 6.170%.
S&P has returned ~6.5% over the long term in inflation adjusted returns. However, the trend has been lower returns over time and inflation is currently very low. https://dqydj.com/sp-500-return-calculator/ (You can look at the methodology, but they project further into the past.)
6.736% : 1871 to now
6.525% : 1900 to now
6.281% : 1930 to now
5.977% : 1960 to now
Further, there is a lot of volatility in these numbers consider:
7.446% : 1990 to now
2.352% : 2000 to now
PS Stable companies reasonably have a premium associated with that stability.
In any case lets use your 7% you end up with 400bill which is still 50billion less than what it is trading at.