- Cheaper than their Primaris counterparts, often significantly.
- Larger variety of units to choose from.
- Prevents wound overflow via only having a single wound in most cases.
- Wide array of weaponry and customization.
- Can fulfill a number of roles on the battlefield.
- More transportation/deployment methods.
- One less wound/attack than Primaris counterparts.
- Less specialized than Primaris units
- Doesn't have powerful, shiny new toys.
- Competes with excellent Primaris units
- Everyone has played against these units for years. Nothing surprising to opponents.
- Few impactful stratagems to use during the game.
- One more wound/attack than "Old" Marines
- A ton of new weaponry not seen before this edition -- much of it excellent
- Lots of very specialized units who excel at their featured roles.
- New and "shiny" unit syndrome.
- Awesome stratagems to use; at least for Intercessor units.
- Increasing variety of units to choose from.
- High prevalence of D2 weaponry in meta exuberates the cost of the 2 wounds for Primaris.
- High level of specialization can hurt them on battlefield if facing units they are not built to handle.
- Not many competitive transportation/deployment options; forces them to walk from get-go.
- Much more expensive than "Old" Marines, especially when reaching for premium weaponry options.
- Very stringent in their customization. Very little options for each individual unit to diverge from their specified role.
- Close combat efficiency isn't there. No specialized power weaponry beyond occasional sergeant or dreadnought/warsuit.
So, like I said, this isn't anywhere close to an exhaustive list of things but gives us a great starting point to analyze these units. The biggest thing I want to focus on is the high level of specialization that comes with Primaris vs our more classic astartes marine. When you take a Primaris unit, it is very obvious what that unit's role is on the battlefield and is often equipped with excellent equipment to help fulfill that. However, you don't get a whole lot of customization with those units and if you do, you pay a huge premium for it (e.g. Inceptors upgrading their Assault Bolters to Plasma Exterminators). Meanwhile, you have tons of customizing options for our "old" marines. You can often take anti-vehicle and anti-infantry weaponry in the same unit, for a marginal increase in points cost.
Why does this matter? It allows your forces to fulfill duality easier if you are using "old" marines. This means that you have the potential to at least have some kind of an answer for every kind of threat opposite you on the board. In a competitive environment, that is critical. With that being said, such duality often comes at the price of efficiency and execution on the board. That is where Primaris marines are superior: role execution. Here is an example: "Old" Marines have a devastator squad of plasma cannons that average 8 shots a turn. Hellblasters are a roughly plasma equivalent, but have 10 shots per turn, albeit in rapid fire range. However, the Hellblasters weapons hit with AP -4, instead of the AP -3 of the Devastators, which gives them the ability to deal with vehicles and heavy infantry much better. They are also able to absorb losses better, due to having 5 weapons rather than only 4 with the devastators. Another example is ability to utilize Concealed Positions. Primaris marines have access to no less than 6 options for Concealed Positions (2 HQ, 1 Elite, 2 Troops, 1 Heavy), rather than the single option for "Old" marines (Scouts).
While having duality in an unit by unit scale is huge, the Primaris units in your army can also fulfill duality at a macro level. What do you mean? Well, you can take specific units in your army to fulfill certain roles on the battlefield. Considering that the Primaris tend to be superior at specific tasks for their points, this can be a boon if you do it on a large scale. However, if you don't do it for everything in your list, you risk leaving holes that your opponents can exploit. Then again, you can create a pretty nice rock list doing it this way.
The other big elephant in the room for this argument is close combat. While Primaris do have an additional attack, it is usually using basic attacks. The Primaris marines do not have dedicated close combat units beyond Reivers and those do not have access to power weapons. They can buff an Intercessor squad up with stratagems, but you still suffer from the same problem: lack of high AP weaponry. "Old" marines do not have this problem; in fact, they have an abundance of units that can be dedicated towards close combat. In addition, Veteran units have an extra attack, so the attack stat advantage of Primaris is a wash. Some of the most scary close combat units out there are "Old" marines and I do not expect this to change in the near future.
So to me, the argument comes down to two things: How much dedicated close combat do you want/need in you list? How much of an "all-comers" list are you looking to build? The classic marine units are still perfectly viable. Primaris marines have a bunch of cool stuff that is highly specialized and effective, but they aren't replacing our older models. In many cases, the points cost of Primaris units are prohibitively expensive to field in large numbers. They are a tool to add into our lists to make them more effective, but I don't see any future where our "Old" marines are going anywhere.