The investors entering the market today will make prices go up today, but have the effect of reducing prices in the future (either or renters or buyers, depending on how to investor chooses to recover his investment). Investment in housing in past decades has reduced the price families need to pay now. There a zero investors in housing who are not out to either provide housing for rent or to eventually sell housing.
>because this isn't what we see happening.
What we see happening has alternative explanations, for example: there are millions of very-high-paying jobs in the Bay Area, which is course is generally a good thing, but it has the undesirable effect of making it hard or impossible for families that do not have one of these very-high-paying jobs to live here because there are strong barriers to the creation of new housing, which forces the people with the high-paying jobs to spend more than want to spend on housing, which (happily for them) they are able to do (because of their high income) but which sadly makes it impossible or very onerous for lower-income families to live in the Bay Area. Most of these barriers are in the form of regulations imposed by local governments. Where I live for example in Marin County, growth is controlled (kept below the level investors as a group would prefer to have it at) in part by denying new housing projects the ability to connect to the Marin Municipal Water District, I have heard.
So you don't think that adding demand without adding supply makes prices go up?
Or you disagree that prices going up makes it harder for a family to buy a house?
Edit: I see you added to your comment...
> so their investment helps out families on net by helping to increase the supply of housing to meet the increase in demand
And I still disagree, because this isn't what we see happening.