How do we expect cryptocurrencies to be adopted as valid payment methods when everyone is really using them as investment stocks?
When you say cryptocurrencies, you are referring to all coins. You should stick to your concern about bitcoin only being used as a store of value. Not all coins are destined to be a store value, you can see many traders that are disseminate their holdings to different alts for the purpose of trading. This shouldn't be a problem anymore as crypto's can be use with different purposes depending on the holders.
Will we only see proper adoption once the current greed and "quick buck" mentality subsides?
The adoption has started due to the support of decentralization though the mentality of what you mentioned was there.
If things continue the way they are, then cryptocurrency will always and forever be destined as store of value assets and not really a currency that can be used in the real world?
Not literally all, few will be used as currency while others will be used as store of value especially bitcoin. Try to see some updates on Japan, it's an example where people are using bitcoin as currency.