More and more people are using iOS devices as their only computers for several reasons. The popularity, expense, and maintenance costs have been outlandishly high for years now. My gf has gone through the iPhone 4 to 4S to 5 to 6 in the past 2.5 years -- and she does not own any other computers. She can't really afford them and always has a broken screen she is trying to save up money to replace. This is the new normal.
Can you use an iOS device for all personal computer needs? Certainly you can, and depending on your pocketbook, you can pick up accessories such as the ClamCase or apps described in books such as #iPadOnly. The first real post-PC book. How to use only your iPad to work, play and everything in between.
Can you secure a personal iOS device against nearly every drive-by threat? Yes, and this is very easy to accomplish if you have a few strong controls (e.g., 12-character-length mixed-case mixed-character password, never use public WiFi, leave Bluetooth and NFC off, never install unofficial apps, etc). Some of these have been described in the NSA hardening guides or via the Center for Internet Security benchmarks.
For protection against carrier networks, I agree that we put too much trust in them. SIM cards, just like USB and SDcards, have been known to contain spyware such as NSA implants (many countries can perform the equivalent). Some devices shipped with rootkit-like technology such as found in the CarrierIQ fiasco. Many cell towers utilize stingray technology (i.e., IMSI catchers), especially near malls, airports, and government buildings. In these areas, you will also find law enforcement use of automatic license plate recognition, facial recognition, and many other privacy-busting technologies. Perhaps these are only used to combat terrorist activity or child kidnappings. It is a bit presumptuous to think that these national security or localized law-enforcement technologies will ever be used against you personally, however, so most people tend to fight their overuse by way of the ACLU, the EFF, or other legitimate channels.
The technology that I use is simple: a data-only SIM card via the Straight Talk BYOT program. You call Straight Talk, order the SIM, and plug it into your GSM (preferably AT&T-compatible with LTE) device. If you are very concerned about your data, or have a specific need for your most-secure data, such as intellectual property (e.g., say you are working on a patent), then you might want to avoid cloud, virtual, and outsourced services. For example, you could setup a private infrastructure (e.g., ownCloud) and pay for insurance (techinsurance.com for the baseline, plus cyber insurance) to cover residual risk.