GDP(Expenditure view of GDP): /Why aren't investments categorized as non-final goods?
Well first of all the story why we only record final consumption is not completely straight - it’s not because we only care about activity of “real person” or consumer. But because counting it all the way would mean we double count the value of the good. 

For example imagine the following situation. A wood is produced and sold for 15\$ to furniture company and then furniture is sold to consumers for 30\$. Well it’s not that we don’t care about the 15\$ because it was bought by company, but because the 15\$ value is already “hidden” in the value of the final good. Counting both would give you value of 45\$ but thats double counting the 15\$ is already "hidden" in the final price (of course final price could be sometimes lower if the second business by mismanagement manages to destroy value - i.e. producing something with lower value then value of the inputs - then you would want to record only the final price still to account for the fact that the process destroyed part of the old value).

Even when business purchases something which is not used for further consumption it counts as final good, purchasing catering for some firm celebration would be reflected in consumption since the catering is final good.

However, while we can’t really count let’s say wood that will later (same year) be used for furniture production we can count spending on machinery because there would not be any double counting since it’s not like you just modify the machine and sell it later as some final good. 

This is because GDP basically tries to measure output at current prices. You can do that either by income approach since by definition output is equal to income or by spending approach because all output is going to be used or “spent” in one way or other. Investment also represents spending the produced output so it should definitely be there.

The same question you are asking about gold plated machines can be asked about any final good. If a consumer wants gold plated PC instead of normal PC is that any less or more rational than business owner wanting gold plated machine? The answer is if you want to assume people are rational the business owner will only invest in golden machine if that’s somehow relevant to business process otherwise it would be outcompeted away by other business. If you want to relax the rationality assumption then why should we assume consumers are rational and business not? If we count irrational purchases by consumers we should not discriminate irrational investment.

Regarding the auxiliary question about depreciation. Depreciation actually is not reflected on GDP because it is Gross Domestic Output. However, there is also NDP - net domestic product that controls for depreciation.