In cameras there are several respected brands to consider. The same for lighting, memory cards, filters, gels, and so on. You will often hear people say they are a Canon shooter, or a Nikon shooter, but is this a good thing? Not really. But there are pro’s and con’s to this, like everything else in life.
On the pro side of the equation brand loyalty usually means you can leverage gear you already bought as you upgrade the core components. In the case of a camera system, and yes, this is all about systems, you have a camera body and then lenses. If you already own a number of lenses, which are very expensive, then simply upgrading the body gives you some new capabilities without needing to re-purchase thousands of dollars worth of accessory equipment. This is assuming the currently owned accessories work properly with the new purchase, but within reason this is usually not an issue.
On the con side, a significant investment into a brand can leave you stranded when that brand fails to keep up with technology and trends. Take, for example, 4k shooting in a DSLR camera format. Canon got there early but has not continued the evolution of the product. Sony, on the other hand, embraced it and ran with it.
Now, in the case of canon and sony, there is a compromise that is quite workable, you can use your canon glass (also true for nikon) on the sony body with an adaptor. So your investment in canon (or nikon) glass is not lost at all.
But this idea, applied to every category of gear, needs to be considered. For example, flash units/strobes. If you invest in the canon system of 600EX-RT flashes and their remote transmitter, they are very nice units. But, they only work for Canon. This traps you. Instead, getting one or two flashes, and then for the power going with another product, like the ProFoto B1 system, lets you enjoy more flexibility. Or another brand… such as Priolite. By going to 3rd parties you open your compatibility up to multiple camera systems while only needing to invest in one set of strobes. The camera-strobe interface might need to be switched out, but this is vastly less expensive than the entire system, and maintains a consistent toolset for your setups.
Different camera brands fill different needs in many cases. Nikon and Canon tend to focus on sports, action, documentary work. They are robust and highly capable in a wide range of applications. They have the largest native lens selections of any brand, and a wide array of accessories for nearly any situation. They tolerate abuse well and deliver years of reliable high quality service. The others tend to be more specialized, or limited, but in their specific niche can deliver superior results. For example, in a safe, stable environment or situation, Haselblaad does very well. With probably the highest potential resolution of any camera, spectacular color depth, if you are creating high level imagery this might be the tool of choice. But these cameras are not quick to operate, and are not really aimed at unpredictable action. Sony, on the other hand, dove into the mirrorless market. A lighter weight camera, since it lacks the mirror mechanism, and has a smaller battery, and is in a smaller body, it tends to be slower to focus, and the range of native lenses for the camera is rather small.
As with so many things in the photography and video world, there is always a compromise. So to protect your wallet you need to stay somewhat flexible. This means not getting too bogged down in brand loyalty unless it serves your purposes. The core system of camera bodies and lenses is where this tends to be the most favorable. If you need to use adapters this will ultimately degrade the final imagery although this degradation, if you bought the right quality adapter, will often be impossible to spot. In some cases, the adapter can benefit you, for example, if you have the Sony A7S camera, and you shoot in APS-C mode (where it uses only the center of the full frame sensor) and you use the metabones speed booster with something like a canon lens, you actually gain light, a full stop (double the light). This is because it is taking the full frame imagery from the canon lens and sending all that light to the smaller aps-c size sensor area, effectively doubling the light it would normally get. Slick, huh? This gives you benefits that would be otherwise impossible. Also, it gives you back the wide angle perspective lost by shooting in APS-C mode.
So, it is all trade offs. Just bear this in mind as you buy gear and try to set yourself up for the best future compatibility.