It is complicated though.
There are two paths to certification for aircraft, the DO-260B way, where the transponder on the aircraft is made to meet the 1090ES standard, and the DO-282 way where the aircraft gets a UAT added. The DO-260B is supposed to be an easy upgrade for mode S equipped aircraft like large transports.
I worked on a project where the desire was to upgrade the GPS on some 737's. The manufacturer of the GPS said no trouble. The 737-NG's have an integrated radio system, where Honeywell supplies the whole package. To change the GPS receivers would require Honeywell to recertify the whole radio system, and that would be expensive.
For both ADS-B systems (DO-260 and DO-282) the GPS must meet certain performance requirements, similar to the WAAS (GBAS) GPS systems. Most new systems are including a WAAS capable GPS receiver in the transponders or UAT and are meeting the standard. For existing systems, there may not be a WAAS level performance system available. On transport aircraft, the IRU may allow meeting the performance needed for the position information. The IRU is capable of being very accurate, since the autopilot relies on this information. Combining the IRU and the GPS may allow the accuracy necessary within the 90% required.
The FAA meant the best
The 2020 deadline for ADS-B out was established as part of the 2010 FAA re-authorization package. The FAA promised to have all the ADS-B ground stations in place by the end of 2013. Everyone thought 10 years would be plenty of time. The FAA mostly made the 2013 deadline. Most of this had been tested in the early 2000's in Alaska as part of the capstone project.
The equipment manufacturers had some equipment available shortly after 2013, and some installations were happening. The road to certification of the equipment was a little slow in coming, and there seemed to be challenges. In 2014 there were a couple ADS-B in and out solutions that were certified, but still very expensive.
The FAA found out they goofed. The certification requirements are not possible on all aircraft. The experimental aircraft are not type certificated. They cannot receive a supplemental type certificate (STC), there is not a certificate to supplement. The FAA had made an exception for experimental aircraft, they don't need TSO'd equipment, and they don't need an STC. Now the LSA aircraft are not certificated either. LSA's must have manufacturers approved configurations to be airworthy. The manufacturers must determine if a configuration is safe and airworthy, so they must test ADS-B devices to allow their customers to conform.
There are other parts of the rules that are proving a challenge. Occasionally an aircraft will not have a good GPS signal due to terrain or buildings. If the transponder indicates a failure, because of no GPS, the aircraft cannot take off. The trouble may not be with the aircraft, it may just be terrain. The FAA is working to address these types of issues.
The equipment available today is first generation. Buying something in 2015 will almost certainly look old come 2020. If the equipment is ADS-B in and out, the MFD and software will probably look somewhat outdated in 2020. Second generation equipment is being talked about already. The second generation will probably have faster processors and more efficient radios, making the first gen equipment feel less capable.
The airlines are in a tough spot. Most transport aircraft were designed well before ADS-B mandate was finalized. The equipment on most aircraft do not meet the requirements, and changing anything will be expensive. The paperwork may take 1-3 years, and then the work may begin. With over 5800 registered transport aircraft in the US, the 2020 deadline looks daunting.
The FAA may offer a grace period
There are rumors coming out of Washington that that FAA is considering a 5 year grace period. The 5 years is what people believe it will take to properly equip the transport aircraft with conforming equipment. During the transition period, the aircraft will be allowed to use the existing GPS and IRU equipment to broadcast the ADS-B position, at a lower precision than will be required after 2025.
The FAA has for the last year insisted the 2020 deadline would not budge. The grace period might be a way for the FAA to save face, and allow a more reasonable deadline.