I guess it depends where your roots lay. I do not like DCS systems as I find they are very restrictive in many ways. Also far too slow for most of my applications. They are fine where scan times and fast collection of data are not important. One of the leading DCS companies, for example, promote their DCS system as something special. At the end of the day it is only a PLC with cheap analogues that probably would not stand up in most noisy indutrial environment. I have seen one used on machinery and it was pulled out and replaced with a Schneider Premium PLC within 6 months. The analogue cards did not like the noise generated around the machine and false readings were a common occurrence. They are fine for building air conditioning control.
For example, an air conditioning system and motorised valves would not care if the response time was 10 seconds as they move very slowly.
Fast positioning of packaging machinery, packaging lines, shutting down a generator that is going into overspeed are far more critical and require fast response times. A DCS would be absolutely useless in these situations. So would Modbus RTU, ASI, Device Net, Profibus etc. You need a fast processor, in many cases, high speed counter cards, motion control cards etc etc.
Some of the networks that are used by DCS systems are certainly not amongst my favourites. BACnet, for example, I find a pain in the tail. It is so slow compared to what I normally use it is not funny. Sit there and wait for 5 minutes to see things happen. But, once again, it is fine for air conditioning.
I do not necessaily agree that DCS systems are more robust and fault tolerant. Dual processor and communications systems are quite common in PLCs, and have been for many years. I quoted, and then withdrew because the consultant was an absolute pain, a hot standby system recently. An option in the quotation was redundant communications between PLCs. I quoted Omron CS1 hot standby PLCs with redundant fibre optic Controller Link cards. You will not get anything more robust or fault tolerant than that.
SCADA systems are very versatile and most can communicate with almost anything, in one way or another. I have installed Citect SCADA communicating with several brands of PLC, fire panels on Modbus RTU, BACnet (that is why I do not like it, have had a bad experience), a modem etc. Most DCS systems have the ability to use BACnet or Lon, Modbus RTU and or ASCII and not much else, unless you roll your own drivers. Citect, for example, has about 150 different drivers available. They are virtually all included with the package at no extra charge. There are a few exceptions, such as RSLinx where AB require a licence fee for each purchase of Citect. Most SCADA manufacturers want to charge you for each driver. OUCH!!!
I might add that PLCs, generally, are so reliable these days that replacement due to failure is a rarity. Most failures are caused by incorrect wiring, a 240VAC voltage being applied to a 24VDC input etc. These situations occur just the same with a DCS system. I hardly ever replace any PLC components.
In most cases, ladder logic is fine and is fairly easily understood by plant electricians. They do have difficulties with function block, structured text, SFC etc. However, many PLCs have some or all of the IEC languages available these days. Probably all manufacturers have all IEC languages under development, if not already implemented. Once gain, depends what you are doing. Function block is great for building control where you write a block once and copy it from zone to zone and job to job. I do not have many uses for it as even my alarm routines vary from job to job and even rung to rung. Sometimes wish I could use it more as it would save me a lot of time.
There are things that can be accomplished better in ladder, structured text, FB, SFC etc. Unforunately there are also a lot of things that cannot be transferred/converted from one programming language to another. Horses for courses.
I think some PLCs are there right now if you require the type of "advantages" that are available in DCS systems. Just about all others will have all IEC languages available in the next few years.
The blurring has been happening for years. The big advantage that PLCs have is that they do not have to run on Bill Gates, they do not suffer from the failure of cheap Chinese computer power supplies, they are industrially hardened, they are designed to work in noisy industrial environments, etc. DCS system are not designed for that.