Speaking explicitly, the value landlord's add is taking on risk so that you can live somewhere you can't afford or don't want to purchase right now. Literally, if they weren't letting you'd have to leave DC right now or buy right now.
Renters, in turn, should acknowledge that they are choosing to take on the risk of being later priced out of buying in a specific neighborhood by not buying now. Especially in times of 0% down mortgages.
Landlord's risk + labor is compensated in cashflow dollars + expected value of appreciation + other (e.g. sentimental value, avoiding the risk renters are taking if e.g. they want to live here 10 years from now, taxes, hassle of selling). If it weren't they wouldn't bother, just like you wouldn't when you buy stocks. The market determines the reward for that risk, for the first two anyway, just like it does when you choose to put your money into securities {or, anything at all}. You've noticed that in some HCOL places it never makes sense to buy except when looking at large appreciation and that third bucket - consider that your friends might even be complaining about people knowingly acting against their own expected value financial best interest to give them a great deal over the next 10 years. That's a deal they'll never see in e.g. the stock market =).
Importantly, real estate is hyper-local, so unlike stocks (where you'd need to work to impact national or global wealth inequality against highly organized groups) your friends can directly impact this market determination! "Landlords" aren't the people your friends should be thinking about, but rather the local laws. These can absolutely be written in a way to deter landlording or buying-to-let, especially by those specifically out to make an outsized profit through less than morally upstanding means or in ways with too many other associated negative consequences. Individual landlords and even buy-to-let companies really, really aren't the people driving outcomes here^. Your friends can directly impact this because it's mostly local or state law.
If they want to do more than complain to they sky* they either get involved politically, come to terms with the fact that the tie goes to the people with money on this planet, or mentally re-frame** the landlord value-add scenario. What if, instead, they thought of it this way: buying prices are what they are, and if there were no buy-to-let landlords your choices would be buy the condo, don't live in an area at all, or, if we're limiting letting not banning it, pay 5x the current rent due to much lower supply (or you're staying in a hotel for 9 months). Maybe without landlords buying to let the buy price is $350,000 instead of $500,000 - would they take that trade, since it would've meant you'd have lived in the boonies that last however many years it took to save up that down payment? And that maybe they'd have lived with their parents until they had any down payment saved up at all? Would that limit high paying city jobs to people gifted the $100,000 down payment at 22? What would that do to society? A fun read can be about the creation of mortgages, which weren't always around (so you'd save up the purchase price not the down payment), and their impact on housing. Basically no societies work this way and it's not for no reason, so we're on to just arguing the implementation details, which are again determined locally.
*there's nothing wrong with doing this, of course. Lamenting the downsides to choices a society has made is completely above board. For instance, many areas where lots of landlords enter see housing quality lower over time (accept this as written for now^). It's okay to acknowledge this as a downside of a society's land-letting laws and lament it without calling landlords evil for taking advantage of those laws. Just the same, landlords, you shouldn't take personal offense at someone saying "you operating here lowers the quality of housing" in cases where it is objectively true.
**This is the third option for a reason: it's a lot more complicated than that, so more to help them be less unhappy than make the world a righter place =). It also doesn't cover the much more tragic cases, imo, of longtime owners priced out directly via property tax increases^.
^There are a ton of interesting, non-intuitive, unintended consequences to various housing policies. If you're interested, there are many, many academic papers discussing the various impacts, eventually summing up to economic policies more broadly and their global interplay. Many of the things I said here might be objectively false upon a review of the literature and I expect that to be true for most people reading this.
As an aside, just as there are obnoxious, whiny tenants playing the "woe is me landlords are evil, why do you get so much profit off my back card" there are landlords who, essentially, complain that there are downsides to RE investments. Losing money (direct risk). Getting called late at night / called out on a message board about <anything, even what's actually something the renter is responsible for, because sometimes you need to educate your customers or your customers don't and won't like you as people (think how a tow truck driver feels everyday)> (lifestyle risk). Or one time that no, I wasn't going to risk injuring myself to carry the new full-sized refrigerator up the 3 flights of walk-up steps for my little old lady landlord like they assumed I would just so they can save $50 on delivery, and now they can figure out what to do about it with it on the sidewalk before nightfall (this is a business transaction and I, as a tenant, am treating it as such). There are also tenants and landlords who do none of those things. Sounds like people to me.
All of this, too, might be more a commentary on the changing global purchasing power an individual person generates per hour in the US than renting, buying, and landlording laws.
